but not individual instances.
You can. If you're using a Lemmy home instance, as you are currently (lemmy.world), in the Web UI, go to your user menu in the upper-right corner, click "Settings", click the "Blocks" tab, and then you can choose instances to block in a panel there.
If what you want is "I don't want auth-left stuff", avoiding hexbear.net, lemmygrad.ml, and lemmy.ml can help. You aren't going to get some kind of ironclad avoidance, but that'll avoid the great bulk of it. Your home instance is lemmy.world. lemmy.world is defederated with lemmygrad.ml and hexbear.net for exactly the reason you mention (in fact, I see people who don't like lemmy.world because they consider it liberal, which they don't like) so you already won't be seeing stuff from the first two instances.
I don't think I've personally seen fascist material on the Threadiverse (though there are some people with quite broad definitions of the term), though there are or were some far-right instances out there, based on defederation lists. Most of what little I've seen on the Fediverse seems to me to be on Pleroma, though I haven't spent much time on non-Threadiverse Fediverse stuff.
moderate conservative
The home instance that I use, lemmy.today, has one user (@DonaldJMusk@lemmy.today) that posts a bunch of Trump stuff and a conservative community, !conservative@lemmy.today. I don't know if your definition of conservative and his match up, but maybe you'd find it to your taste; it's probably the closest to mainstream US, Republican material that I've seen with much activity on the Threadiverse. The instance isn't going to be just moderate conservative and moderate liberal users though. But, if that's the kind of community that you might be participating in, I'd imagine that he'd like to have more users.
EDIT: My own personal take is that the long term solution to having people with disparate positions on what content they want to see, above-and-beyond use of moderated communities and admin activity on instances, is to have "curator lists", where people can basically "share" lists of blocks/subscribes/votes or something like that, and other users can subscribe to them. Then you have a list that
for example
excludes or includes content on various grounds without requiring effort on a per-user-who-wants-curated-content basis. I think that Usenet pretty much established that killfiles don't really scale well in combating spam and stuff like that, because there was never a mechanism to share killfiles among users. Anyway, today, there isn't support for something like that on the Threadiverse. I understand that BlueSky has something along those lines.
