I am not super clear on Mastodon either, but I believe each server keeps its own set of usernames, so I'm not sure there's a point namesquatting there. You would be ratcreature@serverofchoice.yay and there might be another ratcreature at another server. It might be worth trying to get an account on a specific server where all the cool kids are, but they're starting to have issues with the people who run the larger servers getting tired and quitting and shutting them down (which is the obvious fail point of that model) so I'm not sure it's even worth trying to get a foothold on a specific server.

Discord seems to be the main place at the moment tbh, but I don't think it requires unique usernames, so there's no point in squatting.

If I'm trying to predict the future, possibly Twitch? Non-gaming fandom stuff seems to be slowly creeping in there as gamers-who-are-also-fans use it for other stuff, and there's a lot of active people and a thriving visual art community. I think you do have to namesquat on twitch, but I haven't quite gotten as far as signing up yet. Also if you aren't interested in streaming there's really no point.

Tumblr is having a mini-revival helped by them removing the tits ban simultaneously with the twitter exodus, but I'm sure you've already got that one.