I use social.coop, and it seems to work well. They have some kind of participatory administration too that I don't know very well.
My recommendation is to find an instance with some kind of governance plan, and hopefully a stable stream of donations. Volunteer instances indeed could go at any time. And of course, if you can, donate to one or a few instances out of good citizenship.
I think it's also worth mentioning there are other services built around ActivityPub as well (and federation). I found friendica pretty good (as a Mastodon/facebook alternative), although I don't use it.
Instances I can recommend to HN fellows are probably https://floss.social/about, https://chaos.social/explore and https://mathstodon.xyz/about. But don't worry too much, you can move later without too much issue (though moving posts themselves is not possible).