Another thing that bothers me, in Spotify and pretty much everything else: you can't add playlists to other playlists. Like union directories. The most important thing is that it's a link, so every list updates whenever I update the included one.
If there's a program with this type of functionality, lmk.
I don't really understand how that is useful but if you need to do it manually you can just shift click all the songs and add them all to a playlist on the desktop app
You can use the Spotify Smart Playlists feature to do this. I used to do something similar before giving up. It's clunky, but it works. You basically set it to pull all new songs from the feeder playlists into the accumulation playlists, every night.
If there's a program with this type of functionality, lmk.