SuperCollider

domain-specific language unfortunately, but at least it's on GitHub, looks promising.

How to play scores and save them to files is discussed at: doc.sccode.org/Guides/Non-Realtime-Synthesis.html

They have a nice looking IDE, but running anything from the command-line interface is super hard, much unlike Csound. How to get a decent hello world: stackoverflow.com/questions/65360414/how-to-play-a-supercollider-file-non-interactively-from-the-terminal-command-lin

Sample composition with custom synths + notes: sccode.org/1-5cl

leanpub.com/ScoringSound looks like a decent tutorial, it is basically the Csound FLOSS manual for SuperCollider.