> but it's the fastest way I've seen to get from zero to functional GUI
I think Qt Creator is still the best option I've used there. It has a very good GUI form editor (probably the only one I've ever used that is good). You can have a basic GUI up and running in like 5 minutes tops. And C++ is a much better language than TCL, and Qt is a much better GUI toolkit than Tcl/Tk.
Just steer clear of QtQuick and QML. It's kind of trash. QtWidgets is still supported though.
Why would you say QML is trash? QML is absolutely awesome for UI stuff (while still developing the models/logic in C++). I built my own block editor in QML and C++: https://rubymamistvalove.com/block-editor
Except given the way JavaScript and Electron have gone, QtQuick and QML are pretty much the future, as far as Qt is concerned.
Especially because it opens the door to also have Rust in the mix, Slint style.