Having used QML in the past, its made the most sense to me out of all the UI/Animation setups I have ever used. This sounds amazing
Also it is very incomplete. For example there's no way to do custom widgets that draw text (e.g. a graph). There's no good text area widget (there is one but it is extremely limited - you couldn't use it for a log output for example).
Plain QtWidgets is way way better on every level except animation and styling. And maybe touch support.
But if I were making a mobile app I'd definitely use Flutter. Much nicer experience than QML.
1. Refactor out to separate components a lot.
2. Use the .ui.qml and .qml code behind pattern to separate UI from code.
3. Use singleton QtObjects to create abstract classes that contain any non-UX behavior.
As for graphing, the built in components work pretty well:
https://doc.qt.io/qt-5.10/qtcharts-index.html
As far as QML widgets go, they have improved greatly since they released Quick Controls 2 (a rewrite of the initial set of controls).