There is nothing wrong with kbin.earth. Email providers marking mails as spam is not an indicator about the instance whatsoever... The instance is actually hosted by the developer of Interstellar.
Mbin does not currently offer instance migration. It is a long awaited feature, but there are just other things blocking it currently. But even if instance migration was a feature like it is on Mastodon for example, you'd probably still leave your data behind. The thing that is being migrated is mostly your identity and your followers (in Mastodons case).
If you want to be sure that there is no data left behind, just delete your account, that way everything you posted is being deleted from the server.