If that is a major factor use wxwidgets. It is at least reasonably complete

Raspberry Pis have a ton of RAM. Most old ones have 512 or 1024 MB, and new RPi4s up to 4 GB.

If something runs on a Raspberry Pi, that's not exactly a proof it's light weight.

The problem I ran into using Pis was that 3D hardware didn't seem to want to play well with Qt. Unfortunately since using 3D acceleration for UIs is all the rage almost nothing worked without a lot of tinkering (and even then).

I'm pretty sure this has changed since the last major Raspbian update, but the Pi still isn't quite at parity with more mainstream desktop hardware.

okay then, I guess a better data point would be that Java UIs run on twenty year old desktops (500Mhz, 128 MB RAM)