very nice work and documentation. i did a little "plotter photo booth" with a homemade pen plotter many years ago and it was a ton of fun. folks love to see things moving off-screen!
https://observablehq.com/@josephg/simple-sin-waves-plot
I’ve never thought of using a midi controller to noodle with them. That’s a great idea!
But if you look at the commercial pen plotters that were made for doing actual work like HP 7475A - those things fly. https://www.youtube.com/watch?v=Fe9VWvtvNtg It's mostly vintage models, but you can still occasionally pick them up on ebay and other marketplaces for a price that's comparable or better to the new hobby ones. There are also new vinyl cutters that you can attach a pen, but since the main requirements are different they aren't as fast.