I have a good news for you. My Silent Hunter 3 vanilla works just fine on Windows 11 and it worked the same on Win 10 so there is a High chance you dont have to change your PC to older one
(only thing that is still anoying me is that Silent Hunter likes to crash to desktop when you least expect it so you have to save the game often).
Anyway if someone knows where I can find ships from countries I wrote in first post please let me know