It would be simpler just to pick an instance that to date refuses to defederate from anyone. I use SDF, and I picked it in part for that reason.

I also don’t think they’re going anywhere, since SDF has been around continuously since the 80s (literally started as an anime BBS) and still runs a lot of services that would seem unusual to care about these days (including having a gopher server), so I figure their Lemmy, Mastodon, Pixelfed, Matrix, etc instances aren’t going anywhere.