As a long time Plex member, on Lifetime (originally purchased for <$100), PlexAmp is great although not worth anywhere close to $750.

If you're paying $750 you might as well use Roon like the rest of the audiophile freaks.

Jellyfin has a Music Server although a bit limited compared to Plexamp.

Navidrome is a Music Server with similar functionalities.

Symfonium is a Music Player which can connect to various Music Servers like Navidrome, Plexamp, or just files on the network.