Welcome to Subsim.
I'd recommend SH4 with one of the supermods, like Real Fleet Boat (RFB) , or Trigger Maru Overhauled (TMO). You can even play U-Boat captain in the Atlantic with in Operation Monsun.(OM)
I haven't played SH5, but apparently you only get the Type VII U-Boat to command, and it sounds like you need to add a mod soup to get it running in a way that players find desirable.
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