wxWidgets is oldschool, QT has license issues, GTK looks so-so except on Linux, TCL/TK looks ugly everywhere.
In the modern world we need some GPU accelerated GUI library. Something like the one used in SublimeText. But with BSD or MIT license of course.
That would be much more interesting for me.
It's a bit sad that a GUI library absolutely needs to be new and shining to be even considered nowadays, it looks like the whole programming world got infected by JS ecosystem anything-that-is-more-than-3-months-old-is-obsolete mindset.
The old that is strong does not wither.
As someone who has used it and preferred WX over QT for Windows based programs, the issue is not in the look and feel of the final product itself.
It's the heavy use of C style macros instead of C++ templates, mostly.
The WX C++ code looks like Microsoft Foundation Classes. I am fine with it, but for a long term project, this could discourage new people joining the project.
I suggest Slint (https://slint.dev)