Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
I think that the best solution is probably “best practices” and defederatiom used to enforce some sort of minimal Code of Conduct wrt the actual mechanics of running an instance.
Otherwise, the only other way I could see to address this is to lump some data at the instance level. I.e. each instance simply reports a total of upvotes and downvotes from it’s instance, and you just have to trust the instances to behave. There might be some checks to make sure the vote totals are plausible.
In reality, this will be the end of small instances. Only feasible way to enforce this is federation whitelists, and it will be very hard to get whitelisted. Not necessarily a bad thing in the big scheme of things when we weight the positives and negatives, but still sucks for anyone “self hosting” an instance.