If you ask me, where/when historically appropriate milch-cows will make a nice addition. Keep in mind that the first patrol by a Type XIV U-boat started on late March '42, and the last of them was sunk on 12 Jun 1944. During this interval they surely played an important role in the Battle of the Atlantic, especially during Operation Torch, yet you should also consider that only 10 U-boats of this class were in commission, the majority of them not entering active service before summer '42 (some of them much later) and being killed in action between spring and autumn '43. Five out of ten didn't manage carrying out more than three sorties and, of those, three were killed during their first patrol.
On the other hand, before the first "Milch Cows" started their service, and alongside them (after their commissioning) a number of German auxiliary ships - oilers, supply vessels, but also commerce riders - carried out replenishment duties in the North and South Atlantic and in the Indian Ocean, refitting among the other German vessels a considerable number of U-boats. This is to say that completely replacing the Dithmarschen (or any other ships used in that role in the OHII campaing and in the current TWoS campaign) with the Type U-boat, would be historically inaccurate and detrimental to the variety of the game.
If you want to know more on the U-boat replenishment operations that involved a ship as the supply vessel, you will find more details at the following links:
https://www.subsim.com/radioroom/sho...nt#post1945324
https://www.subsim.com/radioroom/sho...98#post1950598
Unfortunately most of the pictures and of the charts that illustrated my lists went lost, but should you need those charts I can recreate some of them. Moreover, if you think that a list of all the U-boats the are know for having been replenished by another U-boat (with dates and locations) might ease your task, I will gladly collect that information for you.
I should have a look into the files. Can you upload them somewhere?