So for a gadget like this the obvious tradeoff is going to be whether you want to lean toward useability (especially if you're only expecting the user to tinker around with your gadget for a few minutes) versus diversity of features. I think you largely struck the right balance here but there are still a handful of really important features that are missing.
First of all, what even is the scale you're using here? It's not diatonic or chromatic. Several times I went to add a note only to realize the scale you implemented skipped over the one I wanted. A single 12 note chromatic scale across one octave would have made this a lot more useable in terms of making it easy to put down the notes you want. You could also label the actual notes (and maybe tint them light or dark depending on if they're white/black keys on a piano) instead of just having colors.
Second, the scrolling is annoying if you want anything of significant length. You have to add notes bit by bit, and there's no way to copy and paste repeated sections or zoom out. For anything longer than a few measures this would be a real hassle to use. I would guess zoom would be a bit harder to do but just an input box that allows you to choose how many measures to add would be a huge QoL improvement.
Last, I don't think it would be too hard to have a couple different instruments you could choose between. Synth, guitar, and piano would probably be enough for a small scale gadget like this.
Really cool idea and the export/import feature is especially cool, but with the lack of QoL features I wouldn't want to use this long enough to create something worth exporting.