I've been aware of Tcl/Tk for a long time but have never really taken the plunge but the following line ,,truly cross-platform GUI's" really stands out in its about page (https://www.tcl.tk/about/).

I have used imgui and it's bindings in multiple languages extensively, any Tk mains can shed some light on how it compares, ie in the areas of speed, devex, tooling and cross-compilation.