What else would "self-hosting all of Bluesky" mean other than a copy of the entire site? If you just want to participate in the network host a PDS, which only stores your own posts.
If you fully want your own copy of everything, then you'd want to run a copy of everything. But you don't have to. It really depends on what your goals are. That's why the post is about the maximal scenario. "Just your own PDS" is the minimalist scenario. But I think it's the one that makes sense for 95% of users who want to self-host.
I feel like the middle ground your talking about could be just a feed?
A feed is: a server that consumes the firehose and decided on whether to store posts, when loaded in the app it returns some post to create a feed
So essentially you only store references to part of the network rather than storing the whole thing