I’ve experimented with ntfy, it works pretty well and is selfhostable. I don’t know if it natively support mqtt though.

I use both IONOS for domains and webhosting and a VPS from Host Unlimited, they also offer webhosting, but I haven’t tried it yet. I’m happy with both. Host unlimited apparently has a tier with unlimited mailboxes.

Volumes are horrible, how would I easily edit a config file of the programm running inside, if the container deosnt even start.

Bind mounts + ZFS datasets are the way to go.

Your best bet is probably PostmarketOS, its not debian but alpine, but does support the phone and aparrently still gets some updates.

Theres a link on the page to the non T variant with flashing instructions.

https://wiki.postmarketos.org/wiki/OnePlus_5T_(oneplus-dumpling)

It should work as long as it can reach the internet

You can’t expose ports on mobile data. You can ether host the web server at home or on a vps or use something like tailscale.