QT is about the only main one that comes to mind.
I have been looking at TCL/TK as that runs on most OS's, but it looks dated.
How does Kodi do it's 10ft interface GUI?
A lot of other GUI stuff has gone commercial, various SDK's you can buy.
UI SDK's for the Tizen OS are interesting.
I have used Python with PyGame,
Kivy extends Python UI's to accelerated..
Netbeans to make JavaFX interfaces.
Recently I have got Ultibo baremetal with OpenVG to work, but that's for my learning about OS's
Mostly based on AJ Starks OpenVG stuff, so try his GO version of Deck?
Android does not use X11 it has it's own way to make screens.
Now that the Pi4 is out, one day we can use Vulkan tools to do this.
There seems to be lot of tools for game UI's that can use Vulkan now.
PiCore is my preferred lightweight Linux OS.
The Pi4 is very usable for native development, time to play with DIY UI/Desktops?
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges