The issue isn't instances.

The issue is that Lemmy isn't designed for interaction with single actors, e.g. Mastodon users or PeerTube channels. Lemmy can only interact with groups, and that's by design because it's a purist Reddit/Hacker News clone.

If you have one account someplace that isn't Lemmy or Mbin (or something similarly specialised), you can connect to and interact with just about everything everywhere.