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)

I use traefik, but caddy should work too. I can share the docker-compose files I use later.

I didn’t understand the docs either, that’s why I’m using https://github.com/girlbossceo/conduwuit insted