(OP here) The main issues I had with them had nothing to do with immediate vs. retained mode, but simply with the unnecessary constraints the big GUI frameworks put on you.

Usually the GUI framwork wants to own the project. I need to build an application IN Qt or IN C# with WPF. Thats just unnecessary. There is no reason why these libraries can't just provide a header with some functions that I can call, and thats it. The notion that I have to use C# for WPF seems completely ridiculous to me. Or that I don't get to use my own build system (Qt) or chose how I want my control flow to happen and so on. I want the GUI framework to be a library to my application, no the other way around.

I hate all of these build shenanigans. I had a C++ application, that I built with a .bat file. I was not gonna port that to C# or have some bullshit wrapper around C#, or port the whole thing to Qt, or switch my build system. That WPF and Qt required this of me is just unnecessary, and I was super annoyed that everybody thinks that is just ok and not a big deal. Stuff like this makes modern programming a slog and kills my productivity and morale.

I don't know about embedded Qt, but after seing that standard dev Qt download is 40 GB, I already knew this is not going to work because 99% of that is going to be stuff I don't need or want, and (as it always goes with these things) it is going to cause a lot of friction, because, of course, I will still have to interact with those things.

So, yeah, basically I just couldn't find a small, simple, non-intrusive, no arbitrary constraints GUI library that looked halfway decent.