Lidarr has a Spotify playlist syncing feature built in. You can set it to sync certain playlists and it will automatically add those artists to Lidarr. I think you can also configure it to monitor/download only the albums which the songs in your playlist come from, or download the whole artists discography.

I can’t attest to the quality of Usenet or private trackers for music as I don’t use either (mostly public trackers for movies and tv) but public trackers weren’t the easiest to get all the music I needed. I ended up using Slskd, a frontend for SoulSeek, with the Tubifarry plugin to connect to Lidarr and it is much easier to find releases now.

The next issue I have yet to tackle is porting those playlists to a player for Navidrome. If anyone finds a simple way to do so please let me know!