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. :)