I realize this is a very broad question, so to help I'll mention that my primary experience with any programming language is Python. I've looked into C and C++ as well, but I haven't written much in them; in part because they're more involved, and in part because I get lost in the IDE weeds with'em (whether choosing an IDE or getting it configured to even get started tbh, but that's mostly a different topic).
In Python I know there's an option in Tkinter, and I've worked with it to some extent but never got entirely comfortable with it. Maybe it would be best to try making some more stuff with it instead of bouncing around different things, but would that be advisable over something that may be better suited to the task?
If it would be better to stick with it, what might be some things you wish you'd known starting out with GUI programming (whether particular to Python or generally applicable)?
PyQt is pretty good and Qt is widely used, so may also be worth a look. The Qt designer is a nice way to separate out the look of the GUI elements, then work on connecting the app functionality to that after. I think the key is to start on something simple and work up, partially to learn the library and partially to learn the pitfalls of GUI development as you add features etc.
Is there something else like the Qt designer for working with PyQt/PySide? I looked on their site and even dug through to the open source licensed versions, but all the available installers seem to insist on making an account to proceed with installation. If that's a no-go, then I may still try it at some point just without that tool.