What you are describing is basically Mastodon (or, if you like porn and hatespeech, twitter… non-consensual porn because a lot of Mastodon instances are REALLY horny).
The moment you aggregate communities across instances you remove the ability to moderate them. Because maybe a hexbear mod wants to remove all mention of the Uyghur people, an ml mod wants to remove all mention of genocide against them, and a zip mod wants to remove all the comments about why genocide is good in a thread about god damned Bluey.
Do they all get to delete everything across every instance? Do you start having different views of the same community depending on your home instance?