The app is made with flutter with mostly just the default widgets that I customized a bit. I'm not really that versed into UI/UX so I just tried to keep things simple design wise. As for performance, I didn't even have to do that much optimization except for the library part to have smooth scrolling when displaying hundreds of diagrams but overall the framework is pretty fast and a joy to work with.

I'd say the hardest part honestly was just staying consistent for more than a year alone without really any feedback and just sticking to it a little every evening and on the week-end rather than playing a game or something. Especially making the content itself was at time a bit repetitive like the lessons or the chords for the library (which were all manually taken from books not auto generated)

I started the app store process quite some time before release so it's just something I did a little here and there in between commits and overall it wasn't that painful.