I'm not the best person to explain or recommend it, but I'll suggest "immediate mode" GUIs partially because I want to hear more about the pros/cons from those more experienced (I haven't ever worked on UIs professionally). It seems these graphics libraries just need some kind of context to draw to, like from OpenGL, and also have a simpler implementation in general. That might make them more portable and smaller, if less native-feeling. I just started looking into egui (a Rust crate) myself.
I love IMGUIs but you have to be careful with the to get them to play nice with the event loop. If you're using it for a game where you're drawing every frame anyways, they're great. But for something that the user might keep running alongside other apps they can be a cpu/memory hog. You can of course only process/redraw when an event happens but that can be easier said than done sometimes.
