Great work! They are so much fun to play with.
I can confirm that on 5.1.5 on Linux one of the brushtips is broken (presets including it are not imported, I can use it, but saving a preset fails). This bug seems to be solved though, as on nightly build everything works fine.
They also work nicely with the WIP “wet blending modes” @SirPigeonz was working on some time ago. Not sure if that’s how paint behaves, but the blending is somewhat cool:
