There’s “light on graphics”, and then there is “completely non-game UIs” which is what QT/PySide6 is about. I’d never choose Qt for anything game-related besides some kind of editor. That being said, I’m curious what limitations you’ve found.
If you actually want game graphics in your game, I would simply choose a rendering engine and plug in a UI library on top of it. One that is retained which rules out things like imgui, and one that is mature, which also rules out imgui and most other UI libs. Unless you have a more vivid idea, this question won’t be easy to answer.
