Thanks for posting, I love tinkering with things like this!
I've been using FlowStone (used to be called SynthMaker) for a long time to build my own VST plugins, and I've recently had a tinker with the patching system in Usine, but it's easy to get bogged down in coding and GUI design with those - they're good when you have a particular end result in mind, but not so good for just letting the experimentation flow in real time.
I'll definitely have a mess with VCV when I have a bit of down-time to see if I can come up with anything interesting.
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