I'd go for SH3 plus the mod combo you listed above.
An excellent introduction to sub sims, and a far fuller Atlantic experience if you're specifically interested in that theatre.
The learning curve is as steep as you want it to be, but the realism options allow you to slowly work your way up. You can change them every time you return to base too, so no problem starting a career at low realism, and upping the difficulty after each mission.
As for each session, they can be as short as you want - sort of. To give you an example, yesterday, I started up, sank one ship with the deck gun, saved just south of Ireland, and quit (back to work, booo). About 20 minutes overall, including load times. Of course the career continues from out at sea when I return. Dip in and out, or do a 12 hour+ stint. Up to you matey.