If an expert upvotes it, doesn’t that give this information already? We can do something like this behind the scenes if we want (which may just involve sorting it differently, perhaps showing a “quality score” percentage in addition to the vote count) without having to worry users with having essentially another way to upvote.
The only reaction I would propose would be a “user went above and beyond” reaction, which is also essentially another way to upvote, but it’s not “this is right”, like the above, but rather “this is amazing”, which would be a nice way to give an extra reward to posts (even if it doesn’t affect score or any type of reputation) and might only need to have a very basic contribution requirement. Although limiting how many you can use in some way would make sense.