The only real advice I can give you is to get a schnorkel as early as you can, and use it religiously.
It limits your vision and sonar ranges, and your speed. On the plus side, it also limits the ability for a plane or a destroyer to get a radar return on you, especially in bad weather.
Don't worry about tonnage. Worry about survival. Usually, you will get the opportunity to nail something in a convoy, but don't stick around. Fire from longer range, then go silent and deep. With some skill and some luck, you will be able to avoid the escorts.
Individual ships still sail, so those are easier to get. Don't try a surface attack, as they almost all have guns at this point. If you *ARE* surfaced, haul ahead of them before you are spotted, then submerge and wait for them to come to you.
Using these tactics, with a career started in January of 1943 I managed to last until December 1944, sailing out of Bergen in a Type VIIC. I was hoping to get a Type XXI, but there is no way you can get the 30,000 renown required, starting that late in the war.
I ended up getting hounded to death by Destroyer Escorts in fairly shallow water (100 meters deep). Should have known better, but I got fatally greedy.