For UI there is https://github.com/kivymd/KivyMD for Material design on top of Kivy.
And the team is nice (I've met some of them at PyCon or FOSDEM).
The framework is pleasant to use, and there is a descriptive language, kv, which is really great.
Cross compiling may be painful though (I did it for Android) and the app loading time is a bit long, but it's working.
Some things may be missing in comparison to big frameworks such as Qt, there is no WebView for instance, and accessibility is unfortunately not as good.
It's overall a very good project and it's a pity that it's not more known and used.