Qt 6 - https://doc.qt.io/qt-6/mobiledevelopment.html

• ubuntu touch UI framework / suru: qt 5 based, so slightly outdated underpinnings, but still actively maintained https://docs.ubports.com/en/latest/humanguide/index.html
• flutter: developed by google as a cross platform framework, tied to the dart programming language. https://flutter.dev/
• Canonical has created custom widgets for flutter, if you don't like material design: https://pub.dev/packages?q=ubuntu
• uno.platform: C# framework for cross platform apps. https://platform.uno/

I have been using Kivy and like it a lot for simple stuff. Its cross platform Python GUI.

https://kivy.org/