Might not be true for all setups, but I find that Plex is a resource hog. Navidrome and Jellyfin are a lot leaner. Plex was using quite a bit of CPU just browsing the library.

Plexamp sounds awful to me, trebly and thin. Googling around I found it was a common complaint.

It DOES handle multichannel audio well, though I don't think it can do Atmos natively.