If you’re proposing overhauling the whole architecture of lemmy to use consistent UUID-based IDs for comments, posts, etc. across all instances, that could probably work but there are some edge cases especially with malicious actors, and it would be a huge undertaking.

That was what I was suggesting yeah, version 3-5 look like it could work, you could use the originating server as the name-space, and a local server generated ID for the name. As long as they only use information sent elsewhere the hashes should be reproducible, so you can check that a server is only using it's own name to send new comments/posts, which should protect against the obvious attacks.

The more I think about it I'm not sure you would even need to use an official UUID system actually, just make something like <originating server>-<id from origin server> as the unique ID?

I agree it would be a big change to make though, especially dealing with all the existing posts.