It’s basically XMPP with a clear packaging, from server to client

Yes.

If you want to access your NPM stuff on both Tailscale and LAN, either:

• Advertise a subnet route for your LAN range, configure Tailscale devices to use it, and use your LAN IP on the AGH rewrite, or
• Split Horizon: Have your DNS respond with a Tailnet IP when it’s queried from the Tailnet range, and respond with a LAN IP when queried from LAN. AGH cannot do this, but other software like Technitium can

Do a DNS rewrite at AGH, but instead of the LAN IP make it the Tailscale IP of your NPM machine. Then configure AGH’s IP address as one of the global nameservers on your Tailscale admin panel

Delete all A/AAAA records on Cloudflare, only use it for registrar purposes and the occassional certs authentication.

Owncloud had a rewrite called oCIS (Owncloud Infinite Scale). Then it was bought by Kiteworks. Then many of the core devs switched to a new product/company called Opencloud. There’s been quite some history over the past few years