You are correct sorry, I was under the impression it was. I'm trying to edit my original post and that is edited, I don't know if that will be updated in the cross posts as well.
Free Software refers to freedom, not price. This "free" is the F in FOSS. Paid free software is still free as in freedom and thus FOSS. Since the free software definition and the open source definition more or less overlap, very little is "OSS but not FOSS" even if it is paid.
I understand this notion, I simply was trying to specify which of these software you can use without paying anything and thus have a lower barrier of entry since it would be inefficient to explain this at the same time.
I checked zrythm recently and haven't personally used it but i will edit the thread adding it!
I tried to keep it to things I've personally tested, but if you have other suggestions I'd be happy to add them.
I got a kick out of seeing helm on your list, as I was using the developer's Unity engine integration for a game when I started coding :) Fantastic synth.
I could also recommend hydrogen as a drum machine - it plays nicely with lmms, which is cool! :) There's also tuxguitar which I use as a guitar sketchpad when I just want to jot down a thing I'm playing, or learn a complicated song. :)