@hinthunter It's not a waste of money to get new players, as even if only some of them are retained you end up increasing the player base. I'm also not convinced that FAF has a poor retention rate relative to other games - given how old the game is, it's impressive that it seems to be growing rather than shrinking, which suggests it must have a reasonable retention rate even if there may be room for improvement. I'm also not sure what increased funding could easily do to help improve the retention rate significantly (whereas with marketing there's a clearer/more direct relationship).
In terms of what kind of marketing I've honestly no idea what would be best, other than that any marketing should be targeted at audiences who are likely to have an interest in rts games (rather than more general based marketing). One option could be finding some youtubers who have a significant following and who post videos on RTS and get them to do a video on FAF, but I've no idea how much it would cost.
With any marketing I'd also suggest doing just 1 method at a time for a period, and making sure we have stats on player numbers by date both for the period the marketing takes place, and the similar period the last year (which I think we already do), to try and get a very rough indication of if the marketing has a noticeable impact/if there's a noticeable spike in new player numbers in the short term following the promotion or event being funded.