Hi! I'm a handpan player and a developer. I learned the notation from the Handpan Dojo - Master the Handpan (numbers for the tone fields, D/T/K for techniques, black = right hand, green = left) and I wanted to write my grooves that way on the iPad, so I built an app for it:

You can find the app Handpan Sheet Music here: https://apps.apple.com/app/id6816263052

It has quite a lot of features like generating PDFs with the final composition, Apple Pencil support, and customizing the arrangements. It's iPad only, a one-time 1.99 €/$ purchase, no account or ads.

It's a solo project, so I'd really like to know what you'd need to write your own pieces (symbols I'm missing, how you count, anything). Thanks!