In terms of niche marketing I make games in unity 2 and see how the low spec the bar can go. I've had a machine from 1999 run some of my stuff aswell asbrand new ones on Windows 11 natively. As for the soundtracks I use third party promotion on youtube and other than that just Facebook and Discord. Maybe the odd online Retro forum. Never found reddit very worthwhile. Just found it to be a bit of a sloppy mess. Maybe I just didn't understand the app. The idea is to get the games to advertise themselves keep the hamster wheel turning. As for advertising to non gamers I don't know. I keep my stuff niche and it wouldn't appeal to most.
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Keeping the specs simple definitely helps. My first experience of playing an FPS game against internet people was Red Crucible 2- there were certainly better games, but that one would work on my low-end laptop and poor internet (I might even have been using a dongle at that point), and it was browser playable through Kongregate and Facebook. Because it was accessible, I played that rather than better games, and obviously, non-gamers are less likely to have high-spec systems so that's a good point to think about.
Good luck with the social media. I'm waiting til we've got at least a beta demo for our next game before I brave that rabbit hole again!