Thanks for checking it out!
Try it on a Steam Deck if you get a chance. 12 to 24 grains is usually plenty for nice synth sounds, especially if you reduce the time between particles. Higher grain counts will push limits and create weird effects, but it's harder to maintain intelligible notes (and much more demanding for both CPU+GPU).
MIDI support and remappable controls were two big features I had to drop to make the deadline. The 'mode' dropdowns (Voice, Grain Input, Grain Position) could also have their own customization pages, but I simplified to those options until later.