> Would be nice to have something somewhere in between the over-simplified ABC and overly verbose MusicXML.

Hard agree.

Early on I actually tried to write my drum charts directly in ABC Notation but it wasn’t a great fit. Then I made a simple parser for my language that outputs ABC because I thought it would be simpler but I found it to be very limiting, so now I use Vexflow’s low level API for rendering. I found it to be more customizable than ABC with a nice JS / TS API. It’s good for my use case (rendering) but ofc it doesn’t work as a save format.

Good luck with Trebella :)