I think downvote anonymity is the bigger part of the problem, not downvotes in general. Unless I'm misunderstanding, what you're proposing amounts to "if you want to downvote in a community you'll need to make an account on it's instance". This would be a nice option to have, but it should also remain an option.
In your +50/-90 example, showing at least the instance provenance for votes allows more (sub)cases. If I can see that 55 of the downvotes come from the instance hosting the community, that's potentially a very different situation than if only 5 do. Or if 70 of the downvotes come from a pair of instances that aren't the community host. The current anonymity of these downvotes flattens these nuances into the same "-40", which I agree isn't great when it can lead to deletion - but I'd argue that's also an entirely separate problem that might be better addressed from a different angle. I find that disabling downvotes from other instances entirely flattens things just as much if not more, just not in the same manner. Instead of wondering how representative a big upvote or downvote count is, I'm now wondering how representative a big upvote count is, period. That might seem like 50% less wondering but with no downvotes at all it might also only be about 50% less votes.
I'm not convinced silencing negative outside contributions won't just shift the echo-chamber-forming to one that's more based around a form of toxic positivity and/or reddit-style reposts and joke comments, either.
Revealing from which instances downvotes come from doesn't prevent opinion downvotes but it allows dulling their bite. The same is true for opinion upvotes.
From my understanding votes are more-or-less already somewhat public on lemmy between it's implementation and what federation needs to function properly. At the very least, each instance knows how many votes they're getting from the other instances. We should embrace the nuances federation brings to the problem instead of throwing them away entirely.
So much thought has been put into "how do we convey the different instances' character and their relations to each other to new (potential) users in a way that doesn't a) overload them and/or b) scare them away with content that rubs them the wrong way" in communities and posts like these, when potentially we just need to render more visible the data that is already present on the instance servers.
I'll acknowledge up-front that the "just" in the previous sentence is carrying a lot of weight; data viz is not easy on the best of days and votes have so little screen real-estate to work with. On top of that, any UI feature that can make what I'm suggesting palatable and accessible to non-power users would also need to be replicated across most popular clients. They're written in a motley assortment of programming languages and ecosystems, and range from targeting browsers to native smartphone OSes, so the development efforts would be difficult to share and carry over from one client to the next. Still, they're called votes: there's a lot of prior art in polling software and news coverage of elections from the past few years that should be publicly accessible (at least in terms of screenshots, stills, and videos of the UI, if not a working version of it to play around with).
On top of this, I don't know how much effort this would require on backend devs for lemmy (and kbin/mbin I forget which is the survivor, and piefed, and any other threadiverse instance software I'm currently unaware of). I wouldn't expect keeping track of vote provenance to prove immensely difficult, but it could cause some sort of combinatorial explosion in the overhead required by the different sorting algorithms proposed (I'm ignorant on how much they cache vs how often they're run for lemmy, for example).
I can't foretell if this would "solve" opinion downvotes on it's own, but I do think it would contribute to the necessary conditions for people to drift away from the more toxic forms of it. It could also become another option for viewing feeds on top of "subscribed"/"local"/"all" + the different vote rankings.
A second good read is her follow-up/response post: Re: Re: Bluesky and Decentralization