I'm in Feb 1943 and have over 60k of renown points

Changed subs IIA->VIIB->VIIC->IXB->IXC, always with the best gear available.
I'm playing 45% realism and always try to get the best possible position for ship attack. With the use of recognition manual's sweet spot aiming it's one shot-one kill most of the times. This earns renown like crazy, especially with the load of torpedoes of type IX.
And yes, I also use deck gun whenever I can, and sink small merchants with 3,7cm FLAK
EDIT: also buying crew for more than 0 renown is a waste. With good tonnages they go up like crazy