a few random thoughts:
Current Gtk is rendering everything via Cairo + Pango. Their future plans is to drop Cairo and use a new toolbox with GL/EGL rendering (see https://www.bassi.io/tag/gsk-demystified/).
Yes, layout is hard. But the hard part isn’t solving the constraints, it’s formulating the problem.
Keep in mind, that both Qt and PyQt are commercial products with some ‘free’ licensing.
PyQt is doing all the heavy lifting between python and a C++ library, it’s not like the effort isn’t spend, it’s just hidden.
A drawing canvas is a rather simple thing, for any interactive UI, jou need a responsive canvas or a scene toolkit.
Hardest thing (speaking about hard problems): Find people to work on this. The ratio of users to developers is close to inf.