Just use Tk. Or wx. They're both well supported, cross platform, and if you use ttk, you can even have your app look good. And native.
For WM, I use i3. If you don't mind tiled, I'd highly reccomend it. It's great, the docs are great, and the config file is easy to understand, so after the base setup, you don't have to worry about it.
XFCE, TWM and sawfish are pretty cool, if you're not into tiled.
Yes, Tk just works for gui and is underappreciated.
wx's default backend is gtk (2 or 3) and they have an experimental Qt5 backend now, which is unsurprising given GTK3's state. So wx doesn't help, unless you can make the Qt backend work (Qt5 is so modular that it's often a pain).
For WM, I use i3. If you don't mind tiled, I'd highly reccomend it. It's great, the docs are great, and the config file is easy to understand, so after the base setup, you don't have to worry about it.
XFCE, TWM and sawfish are pretty cool, if you're not into tiled.