You could check out uYouPlus then, it doesn’t require jailbreaking or anything but instead works through “sideloading”. I’ve used it for years and it’s honestlygreat:
There was actually a project on github that claimed to allow downloading of Spotify straight from the source with drm bypassing called XSpotify but it was taken down by spotify before i got to try it: github.com/meik97/XSpotify
However it was archived on the wayback machine: web.archive.org/web/20200221013751/…/XSpotify
Obviously take this with a grain of salt, you should probably read the source code before attempting to run it