This runs into my objection that, to put it glibly, #ActivityPub isn't actually decentralized but rather re-centralized around instances. And I don't think that's the better way to go.
I think it's better to have actual decentralization, down to the user, as so many engineering and social problems can be solved that way.
You say, "I’m part of a large number of different social networks," but that's not quite right. Since these networks are fragmented it's more like there's a different you that's part of each of the networks, fragmenting you as the networks fragment, which undermines value and your abilities to engage with them all.
In the end, what you're appreciating here is a matter of UI/UX, but you're seeing it as fundamental to the foundation.
Better that we have full decentralization and work on UI problems on the UI level.