IMHO, I'd stick with FotRSU until you really feel competent in skippering a sub. There's enough variables in the sim to make it very difficult.
As others have mentioned, TMO is a
major bump in difficulty. AI is very intelligent! I played with it for a while until I accepted the fact it was outside my skill set. Sims that are so difficult to play aren't much fun!
FWIW......
