Instead of asking for a ntfy alternative, could you tell what you want to achieve? Because ntfy has several functions, primarily that of carrying notifications. And it seems that’s not what you want to achieve. Maybe you can find it here.

Setting up an xmpp server is not a good entrypoint in the self-hosting space. It is quite hard to build. It is a complex system made of different components in order to achieve what you expect from it. I strongly recommend using existing servers. If you want to tinker, find something else (a samba share and a vpn on bare metal is a good starting point in order to learn).

I also prefer xmpp, which I find more stable and easier to set up and maintain.