Welcome Aboard!
There are plenty of downloadable online resources and all of them have useful information. A lot of the stuff on tactics was primarily written for online combat against human opponents playing different [Sonalysts] games, but tactics are tactics. As well as the Sub Command Tac manual, there's also a couple out there by Kapitan about Russian subs and under-ice ops. Then there's
THE BLUE BOOK OF SUBMARINE OPERATIONS
S e a w o l v e s U n d e r s e a W a r f a r e M a n u a l
I didn't write it that way for effect - it was a c+p from the title page
Then there's The Kilo Doctrine. By TLAM Strike - Excellent work and a very interesting platform to play.
I don't have the links for any of these any more, but to be honest, ten minutes of Googling will throw up all of these and more - There's a P3 tactics manual out there that is well worth your time and also a guide to the FFG, which is a very challenging platform, but on the other hand a very lethal one when you get the hang of it.
The mods you play are a matter of personal preference - or you could do what i did and multi install. I found that a lot of missions written for stock, became a lot harder with LWAMI installed because they would often begin with me well inside the detection envelope and making more noise than I should, so you may need to change the starting positions for some of those.
smaller mods that I've found useful are SubGuru's corrected helo dipping doctrine, which as the name suggests gives an FFG player a helo that actually uses its dipping sonar, which it doesn't seem to in stock. another one gives the playable seahawk 16 sonar grams instead of 4 - it's a trade off, but a minor one - so minor in fact that it's never affected me in any way - hence I can't remember what the cost was, for a vastly increased sonar capability.
Hope this was of some help...