AFAIK, Mastodon actually did gain a sizeable amount of users that actually stayed, even though the number of users has dropped since the peak.
I personally think something like Lemmy works better than Mastodon, since content is more important here than the users, which I think makes it more easy to have a self-sustaining community.
It completely depends on the games you play.
Personally, I'd just go with the higher-resolution display, if you notice your GPU struggling at that resolution, you can still turn down the render resolution of the game, but you'd still be on a larger monitor.
The larger issue when gaming is probably that some games don't play nice with ultrawide monitors, so your mileage may vary.