Some new ideas. I haven't implemented them all.
1) Compile your SH playing hobby to a realistic career. Consider where you are located with your skills and knowledge. Cadet>IWO>CPT>Commander. Divide the time you spend on playing, let's say 5 hours a week to priorities as per your rank. This is patrol/theory/studies/writing reports. Details below:
Cadet would spend large portion of time studying Uboat Commander's handbook, type II, VII or IX technical data, history/stories of Atlantic War and biographies by successful U-boat aces. Learn morse, enigma, navigation, mathematics... Maybe try individual things in SH/Wolfpack like navigation, TDC computer and diving.
IWO could start to participate in short patrols. Preferably playing Wolfpack as part of crew, but you could also try SH Captain and practice with Type II in Baltic Sea by identifying neutrals. Focus especially on tutorials. Balance between patrols and theory 50-50. Focus to developing your own routines for TDC, identification, navigation and such, so that you know how to apply the knowledge ingame. As when attacking convoy every minute counts and then it is too late to learn basics.
Captain is probably where most people are located. You spend 75% of your time on patrols. The rest 25% is spent at harbour, depending on U-boat/crew damage-repair and earned vacation time. That time (week or two real time) you can spend to take a break for the game and spend on compiling logbook, reports, learning from your last patrol and also new theory. A U-boat Captain never stops learning (or if he does, he won't live long as the enemy keeps improving).
Commander is the phase where you have seen so much that you would stay on land and reduce your patrols to zero. However I think many would not enjoy this, so I think that it is fair to keep them at 25% to represent how Dönitz maintained his touch to patrol life by interviewing Captains after their patrols. So this is how you maintain your touch with the bread and butter. The other 75% time you would spend at the HQ. Depending on your department is training/tactics, operations or research. So you would spend time on modding, research, writing tutorials and guides for new Captains and perhaps leading Uboat campaign and interviewing Captains like Dönitz.
How does one determine and progress from one level to the next, I think it is entirely a personal decision with no definitive answer. You can base this on years played, tonnage sunk or just on your gut feeling. If you are KIA on patrol, maybe you want to start all over or just downgrade one level (eg. CPT>IWO). It is not impossible to drop from Commander to Cadet and start from ground up, because you are always learning new info and it is best to keep humble. Again this is an entirely personal decision.
2) Another progression of career is U-boat upgrades, new torpedos and crew improvement. The game handles this as renown, but you can make up an alternative, again based on time, tonnage and other factors. For example the new type III torps are on limited supply for the best Kaleuns, so you can only requisition them after 200 000t sunk on campaign. Also if you do badly, eg: Fail to meet objectives, monthly tonnage quota or mis-ID+sink ships such as allies and neutrals, you will receive detriment, such as loose upgrades, special torpedos, specialist crew or even get downgraded from Type VII back to Type II. Since it is hard us to make such choice objectively, you can do it with random generator or scale it with your mistakes before the patrols. First time you might just get off with a warning from Dönitz.
3) Medals and ranks
https://uboat.net/men/ranks/
Research how these were given. Again, you can do things differently than the game tells you. If you have earned your medal, perhaps it is possible to order replica from Ebay or similar. Or these days use 3D printer. You can them put them in some container by the fireplace and remember your adventures and kamraden when you are old.
4) Make a routine for your patrol planning and debriefing. It is just as important as the patrol itself. Plan your patrol on Kriegsmarine Grid Chart, plot down air/mine threats, convoy routes, measure distances, plan navigation schedule and fuel consumption. Look up sun/moon almanac for the season. Do you want to leave harbour day or night? What about passing enemy air bases? During the patrol maintain your logbook. It is the single most important source of detailed info.
After the patrol write a report to BDU. With basic factors eg. How did patrol go, were objectives met. What did we sink. How was the weather. How was enemy activity and skill. What went well, what went bad. What can we learn. Finally report when you are ready for next patrol, considering damage to uboat/crew and need to rest. Adjust time spent at harbour by these factors, so that it is not always random/28d but scaled rationally. I also warmly recommend making a map of your conducted patrol route, with dates, contacts and main events. U-boat commanders did this and it helps you to put individual encounters to a larger context. Keep these organized digitally/physically in their respective patrol folders. I also did a map of a convoy battle to better understand escort position, behaviour, location of neutrals/big ships and tactics employed.
This all might seem like a lot of work, but by doing this you write down the story of your patrol. So that all the effort and time you put into it is not immediately lost once it is over. Instead you build on it and can go back to see how you handled problems before which you will face again. You learn new and gather intel to plan your future patrols better. One thing Kaleuns often are on lookout for are new possible convoy routes. If you encounter convoy in a place without known convoy route, plot it down and then you have knowledge of new hunting grounds.
For planning you can also make a crew and item manifest. Who do you need, with what speciality. How much fuel, torps and food do you need. Food can be simulated well, if it is your personal rations for next month or two (see part 7).
If you simulate crew, they will get wounded or MIA/KIA. Write a report on the circumstances to BDU and families. Sometimes crew will just transfer. Also when you have medals for them, write a request to BDU who has earned the medal and why. If someone comes short of expectations, you can dismiss them.
5) Other useful material for patrol:
Make a checklist if you keep forgetting the hundreds of details. How to conduct convoy attacks, plan and prepare patrols. It is no shame, pilots and astronauts use checklists to minimize human error.
Make a target card, lots or single laminated, where you can draw ship features (funnel, hull, island...) as you ID it. Once you have ID enter parameters such as height, depth, course. That way you have the info at an arm's lenght when you are setting up TDC.
Research and make a powerpoint of enemy threats. Mines, destroyers, aircraft. What kind of armament and detection systems do they have per year. What are their strenghts and weaknesses. What kind of experiences have U-boat Kaleuns have had previously with them.
6) Adopt the tactics of an U-boat Ace. For example I have learned from Otto Kretschmer to prefer surface attack and use 1 torp per 1 ship. Wolfgang Lüth had a good view of maintaining morale during long patrols by arranging routine/events to crew and also to maintain constant study and self-development.
7) Bring life around the patrols by gathering news/music/speeches of WW2 and arranging them by timeline. Once you are bored on your patrol, follow those as their date, to know what is happening in the war around you. Even more important are BDU standing orders and situation reports. Read these for your patrol and apply the operating procedures (sinking restrictions, torpedo settings) to your patrol as per the date.
http://www.uboatarchive.net/BDUKTB.htm
8) More realism is gained by visiting museums. Especially U-boats, U-boat pens, war/ops rooms and escorts. You can also build small scale model U-boats and ships. Then you will begin to understand the scale and machines. Again, maps, manuals, books, internet, archieves, serials and movies give you perspective and added knowledge. You can construct a personal archieve/library for this information. It can be just one shelf or few folders. It can also be in digital format. Share information and material with others. They might have something you don't and vice-versa.
9) Greater realism can be achieved with food. You can combine U-boat life with the diet you have always wanted! Early on the patrol, you can enjoy fresh food such as sausages and fruit. Later on the patrol you are left with canned goods. If you can safely simulate taste of diesel/mould with spices, that is extra bonus, as U-boat crews often complained about that. If you are getting sick of canned goods, maybe you can go fishing at a nearby lake or supermarket, and catch a type fish which is common in the region you are sailing at. Maybe the Captain also has secret stash of cognac for long patrols and to give some warm feeling to the watch crew at 3 am. If you have a wife or significant other, you can agree to write letters from the sea.
Once you return to the harbour for a week or two, then you can let go of all limits. Feel free to go to a nice French restaurant (similar to Lorient) with wife or significant other, as you know you will have to go back to sea in a few weeks. If you have sunk over 50000 tons, get yourself a good bottle of something. You have earned it and live only once.
10) If your patrol goes south and you are taking flooding, go to a shower/pool/lake and continue the game wet. You can also take salty water to nostrils with a straw, it feels awful but has positive cleansing health effects, and gives feeling of drowning in salt water.
If you take damage and you need to repair it, go repair something with your car or house. You can combine this with an unplesant chore and suddenly it becomes lot more fun. Maybe add time pressure also: If you don't fix it in 15 min you sink. Just remember not to combine electric tasks with water!
11) Take screenshots of the best game moments with old-fashioned photofilter. Attach those photos to your reports and make a photoalbum of your patrols in the good old days. It can be next to your medals by the fireplace... (part 3)
12) If you know a place to print large posters, you can print the large Kriegsmarine Atlantic Chart (and others) and frame it on your wall. Make a small uboat symbol out paper and attach it there with piece of tape. Now you have your very own OPS/NAV room! You can always check where your U-boat and your friends are on patrol. If you get the scale and right tools, you can even navigate with the chart.
Below it attach a screw or two to the wall. Get an old fashioned wooden writing pad mount it there. Here you can keep your BDU orders/logs and other data at hand and walk to it during the patrol. It is like the navigation table on the submarine. If you want to write your reports the old-fashioned way, get a type-writer. One can only dream of constructing an own Enigma-machine...
13) When you choose your U-boat number, check that it is available as per the date. Look up it's history when has it been constructed, has it been to patrols, who has commanded it before and does it have any special features. Read the Captain logs to notice these special features or damage you should be aware of. Once you get sunk, see if you did better or worse than the U-boat historically or it's Captains. You can also plan your own Captain Emblem (maybe it is possible to mod to game?)
14) Research the story of the convoy you just hit and even the ships which you have sunk.
http://www.convoyweb.org.uk/
Some things that would be cool, but not sure how to simulate them:
1) This was in previous page: Research story and load of each individual ship you sink. So ships in game rarely display their name. As in previous page I can find "ClassName=NKPF" but I have no idea how to go forward with this information.
2) Historical handbooks are difficult to find. Many links are dead or they are rewritten in online text format and not original scan. Navigational charts are difficult to find with sufficent resolution and quality.
3) Use weather history for planning. I haven't been able to locate weather data on the Atlantic 1939-45. Ok maybe this is partly useless as I have understood that weather in game is random to a degree. But of course all weather forecasts can go wrong. So historical weather data and charts could be used in patrol planning for realism purposes.
4) U-boat's cook book? Does anyone have it?