Have you considered trying Godot? I read an article recently about creating GUI apps with it[1]. I've used it quite extensively for games and I'm pretty tempted to try app development with it. The GUI system is a little hard to wrap your head around, but there are some pretty decent tutorials online for that. I got a basic demo running on the pinephone using frt[2].
1. https://medium.com/swlh/what-makes-godot...cfb941df3b
2. https://github.com/efornara/frt
1. https://medium.com/swlh/what-makes-godot...cfb941df3b
2. https://github.com/efornara/frt