That doesn't seem too bad, I agree. Maybe that's why QT is used. I haven't really used QT, but the more modern Windows apis, vulkan, etc all are pretty complicated.
FWIW, vulkan is not a GUI library; if you're reaching for it without a clear understanding of why you're doing so, yeah, it'll seem like a very complicated way of doing things.
Vulkan is a graphics API, not a UI library or framework. It's way lower level and if your goal is to make a user interface, you're not really supposed to do it with Vulkan (but you could i guess)
Yeah, it doesn't look too bad on the surface. However, Qt is bad and I'd advice moving away from it if you can. This is based on ~20 years of experience using it in various projects.
The licensing is trying to paint a picture of LGPL being somehow uncertain/evil, the way Qt advocates non-idiomatic C++ practices - I could go on for days, but shall digress.