I don't quite understand why you don't just run everything (Plex and the *arrs) on the Beelink. It's plenty powerful for that. I know, because I have the same and it works great (although I'm using Jellyfin).

For the OS I recommend Debian. With the latest version (bullseye), I never had any problems.

For me the biggest is probably Jellyfin. Before, I needed to use external drives plugged into my TV, then browse them using the TVs file browser. I didn't see which movies I already watched, or at which episode I stopped. When I wanted to watch something on my computer, I had to get the drive and plug it in there. The same for when I wanted something new. Now, I have Jellyfin running on my server, all the clients have access to it and I can watch my stuff whenever and wherever I like. It's also easier to share something.

Jellyfin! It was the reason why I got into self hosting in the first place.