Great writeup. I went through a very similar journey. Signal as the "good enough" default, then gradually wanting to own the whole stack. The bit about smacks and cloud_notify being essential for mobile is spot on, that was the part I always underestimated in past XMPP attempts. Without those two modules the experience is terrible on phones and it's no wonder people bounced off it.

One thing I'd add: if you're already running Caddy as your reverse proxy, you can use its on_demand TLS to handle certificates for the upload and conference subdomains automatically instead of managing them through certbot separately. Saves a moving part.

Curious how federation has been in practice. Are you actually messaging people on other servers, or is it mostly just you and contacts you've created accounts for?