I don't have an answer to your actual question other than self-hosting. But, I doubt you'd ever get a "firehose of trash" on your main feed. Mainly because the trashy instances aren't that populous, so I doubt their posts would be active or hot enough to float to the top. But I could be wrong.

You could always go directly to the instance you want to read and lurk. Unless you make an account you won't be able to comment, but you'll be able to get your firehose of trash

Edit: Just be mindful of how much you consume, you'd be surprised how easily hate can invade your mind. To this day I still have to self-censor every now and then thanks to the hateful rhetoric I grew up around, even though I'm long past it

It's worth noting that defederation-zero is not a practical requirement. There are some instances that just create loads of accounts and spam communities and reply to comments in communities that you are subscribed to. Of course for most instances they don't support this and moderator prodding can solve this. But some instances are simply hostile and exist only to spam.

I do agree that I would like my instance to be defederation-minimal. Unless the other instance is being actively harmful or refusing to moderate at all (for example not retaliating for users who are breaking community rules) I would prefer to be able to see their messages and communities. For example I don't personally agree with blocking Instagram Threads by default. Unless they show complete inability to moderate I would prefer to have the option to subscribe available. Especially because it allows an easy pathway for people to move from Threads to other fediverse instances (although maybe less relevant for Lemmy as it is more discussion oriented rather than personal profile oriented).

I would also like to be able to see per-user instance blocking. This would be a good tool for quick fixes to poorly maintained instances or instances who's values I don't agree with without affecting others. It could also serve as a useful signal to the mods. If dozens of people have blocked another instance maybe it is actually quite harmful and should be defederated.

I'm on an instance that defederated from only 3 other ones: voyager.lemmy.ml, ds9.lemmy.ml and enterprise.lemmy.ml - just checked now. I honestly dont see a firehose of trash, tankie invasion or anythig of the sort. I do aggressively block communities I have zero interest in, so it might still be happening there, but I personay don't see it at all - anecdotal evidence, I know, but that's been my experience.

Fmhy only defederated Exploding heads and Lemmygrad, everything else is cool. And before they blocked exploding heads, it was completely ruining my feed.

burggit.moe and exploding-heads.com

It’s not enough to be federated with an instance in order for the content to pour in. Someone on your instance also needs to be subscribed to the individual communities.

I have a bot account that regularly polls the top posts on the five largest instances. If it notices any post from communities that it is not already subscribed to it does so, pulling all the content from that point forward. Even after doing to this for weeks I’m still only pulling about 800 communities and the signal to noise ratio is still good.

Vlemmy was a good one, although you are almost two weeks late with your question since it fell down :( (Although it was going to defederate with an instance the day before closing due to not wanting to host illegal content, for obvious reason)

I would love for another one to sprung out, I myself am quite trigger happy with blocking people and communities (even instances through my app) so I would always be happier being the one shaping the content instead of another person doing it for me, regardless of if we would block the same instances at least I would know what I have blocked.

I would probably never browse the local feed on an instance that intentionally never defederates though, the potential to jost a plethora of unsavory people is too high (although that hadn't really happened with vlemmy, that instance was insanely good at that)