Interesting... I might still be getting the concept clear in my own head but that can definitely work. It does seem to me that it will shake out with larger communities still under the control of a specific instance, like, I'm sure there will end up being one big canonical Rust community, cause nobody wants to check multiple forums about the same topic.
There have definitely been cases where I did attend multiple forums of the same format and same topic, but generally because I was in transition from one to the other, such as when alt.music.ween was dying and I started trying out the popular Web Forum of the time. In the end I gave up the community of Internet Ween fans, because coming from Usenet, which was absolutely beautiful if viewed using a good client program, web forums were like trying to write a business letter in crayon.
For me, it was actually the first round of enshittification, and we did it without any VC help at all; fact of the matter is that the web forums were easier to use, and so more people came to them, and up to a point, that does result in better community, particularly with non-technical subjects like obscure bands with cult followings.
But, it really did boggle my mind that anyone could use those things without going completely insane at the terrible bad and no good interfaces with no filtering or thread view etc etc (especially early on), but it was an important lesson in psychology for me: they did not have the context of the only-a-tiny-bit-harder, but galaxies-better thing that already existed, they only saw "here is a thing I can use to talk to people about a thing I like, which never existed in my world before," and from there they can tolerate any shortcoming, because it's all novelty. People thought Pong was mindblowing.
Eventually, as the ever-growing blob of spam goo make the Usenet landscape radioactive and impassable, and there was clearly a huge party going on in the forums, everyone just left, other than the diehards that always stay out of some misplaced stubborn urge. A lot of those forums hadn't even figured out how to pay for themselves, and in many cases never did, they just did it so that something would be there where before there was nothing.
All in all, things are really starting to look like the old internet, and it warms the cockles of my heart, truly it do.