Run it in some place where it can easily read the logs of your apps.

You can. But it will be easier if you have a static IP.

I’m not fond of what makes it easier to set up a homelab. Because it might give the impression that you can run it with very little skills. And that opens the door to making your setup a netbot.

I use Radicale as a Docker container and it runs fine, with no maintenance apart from upgrading the image once in a while.

I presume that its power consumption is not to be neglected. Do I’d just keep it off of I don’t really need it.

For very simple notes, I use Memos. It doesn’t have whistles and bells, and it works great!

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 couldn’t think about leaving my personal data to the Big Tech…

I’d add Crowdsec