Sorry to hear the app isn't to your liking. It's made in Pico8 and there's only 8 'instruments' to choose from. I choose the 'best' one, but i can understand that it's not pleasant.
It is possible to make/define new 'instruments in Pico8', and i am thinking of turning this into a full app (without the limitations of the jam). So i will look into that then.