designing a vote weighting system that favors similar instances

Would make the whole thing even worse, as I could create several new instances with 10 bot users each, then hammer out the votes.

The entire problem is that you can’t trace back each vote to a genuine user. It would be bad in case of fake instances that create 100 user accounts and upvote/downvote stuff, but you can ban the instance. It would be a disaster if a big instance creates fake votes (like lemmy.world suddenly adds 1000 fake users and uses them to manipulate other instances, if votes were anonymous you couldn’t check if it’s genuine lemmy.world users or fake accounts).