Agreed that there’s no all-in-one solution to play local music and music on Spotify (if I’m a premium user). I vaguely recall there’s a solution to automate playing Spotify music and record it in real-time (since you cannot download music directly) but it seemed too troublesome, so I eventually chose spot-dl4 to download music from YouTube using Spotify playlist, then the folder got imported into Lidarr/Navidrome, then my Symfonium on Android connects to Navidrome to get the songs.
It’s quite a bit of manual work to add songs to a separate playlist if I like something on Spotify then use spot-dl4 to do the download. At least, I successfully keep a copy of my favourite songs on my server.