The central enemy of reliability is complexity.
― Geer et al., 2003
This did not sound very appealing and would have been quite a bad experience and way to deal with that imho.
However, I gave it a try and reached out to Spotify Support via their contact form: As it turns out, it was super easy to have them move all my data (playlists, as well as “My Music”) over to a fresh account and delete the old one. The support was very, very helpful. Clear information and instructions and very fast. I have been a bit disappointment with their software quality from time to time, especially on Android. But moving away from an account bound to Facebook to a normal account, just using Username and Password was a surprisingly easy, fast and pleasant process.
If you have a Spotify account created via Facebook Login and want to get rid of the linkage, I really recommend opening a support ticket and get it done easily (despite the support FAQ stating otherwise).