Yes: don't. It's very hard.
More seriously. There is very good reason to be encouraged about wgpu as a cross-platform low-level GPU drawing interface for UI work. There are already successful UI toolkits using it, of which probably the most impressive is iced.
For 2D graphics in general, a good starting point is Diego Nehab's 2D graphics course. It has links to a bunch of resources including papers on GPU rendering.
Of course, I'm working on my own stuff, which in time I hope will become a very strong foundation for building things such as UI toolkits, but it's not ready yet, and of course you're already aware of it.