Exactly that thread, lot's of good information there
One thing about renown: Like getting new commands(new subs), refits also seem to depend on renown as Duci wrote in that thread. You don't, however, need to "save" renown to make things happen. In the case of new subs and higher ranks the game looks at your
accumulated renown when deciding whether you deserve better or not. That means you are free to spend all your renown on upgrades and torpedoes right now or save it to spend it on them later, but there's no need to save it in order to make progress in the game. There might be other factors too, like the amount of patrols, but accumulated renown is definitely a major one.