Well you’re in luck, there’s the Pleroma and Misskey family of apps out there that are ActivityPub compatible. Pleroma also has Akkoma as well, and there are far too many Misskey forks to count. Both of them support S3 I believe.

My hope for XMPP is that new and better clients emerge than the ones that already exist. Matrix has this same problem as well.

‘Matrix-based’ is now specified as a requirement in massive public and private sector tenders

I’m pretty sure that most people beyond the tech field know what Matrix is. At least not by name