When I go on vacation I prefer to keep my smartphone with WiFi and mobile data off, but I really don’t like the way Spotify handle offline content. Most of the time it doesn’t download everything and when I do a research, it show me even content that’s not available offline (how can it do that?). Is there a selfhosted service that I can use to download my playlists and play them with an Android app that can download them?
I’m not sure I really get what you are looking for when you say “selfhosted”.
Yes there are self-hosted music platforms, but you bring your own music files.
Maybe you are looking for something like Spotube (available on F-Droid) that let’s you download music (I believe it uses Spotify search then downloads the music from YouTube).
Or maybe you just need to find a tool to download music from Spotify (like zotify) then put them into a self hosted music platform like Navidrome or Jellyfin?