About the Signal protocol: It’s end-to-end encrypted. It would not allow for the server to give you a customized feed. Also messages expire but I’m not sure if that’s to the protocol.

Those cost money too

External communities are just searchable, subscribable and browsable from here. Sometimes you need to change a search filter or default view from Local to All. Or is there something else you feel is missing? I think 90% of the issues people are having are UX related and not a core issue with federation or decentralization.