I'll have to look into how the flow with Authelia exactly looks like. I guess the forward auth flows of Caddy and Traefik shouldn't be too hard to cover. And that's something I'd also want personally: I'm using a similar forward auth mechanism using Traefik, that I currently have turned off because of the app.
The docs of the other integrations of Authelia I have not looked into yet.
One question though, my paperless-ngx is behind an SSO login (I use Authelia) with 2FA. Would it be possible to make your app work with that?