There really isn’t that much when it comes to customization. Your reverse proxy needs to support websockets and I use Caddy for that.
I also host ntfy on the same server and FMD had problems sending requests to it. All I had to do was to allow FMD container to send requests to the host machine by adding ntfy.mydomain.com:host-gateway Docker host mapping to it.
Lemmy (or Fediverse) is not private at all.
Its main appeal (at least for me) is the non-profit motive and censorship resistance, and that you can also browse it anonymously.
If you are getting IP blocked by your instance (server you are connected to), just choose another one.