I thought about writing a cross platform GUI toolkit, that would start from being a cross platform accessibility library.
Installing a screen reader like NVDA and observing how it interacts with the screen might be interesting to learn about the scope/depth of the accessibility APIs in general. (And then there's observing how screen readers handle different websites...)
Imgui and similar generally have zero accessibility support whatsoever, although it's certainly not impossible to add - just a bunch of messages back and forth to the accessibility layers in the OS - but the support would wind up being a module, and a bolted-on one at that that would have very little uptake because everyone who didn't need it (for themselves) would just compile it out. Whereas with Qt and Electron it's kind of like rabbithoooooookaybackawayfromtheinternals and everyone just leaves all the bits in :P and the support just comes along for free. (An interesting study in the tradeoffs between minimalism, modularity, and the commoditization of support of niche use cases...)
Also, regarding practice of UI architecture and optimization, I actually sometimes think about the idea of drive-by contributing to open source projects - they have pre-existing contexts and conventions that must be adjusted to (an important skill that is tricky to develop alone), and it's possible to approach them with an objective impartiality that is sometimes tricky to square with emotional investment in personal projects. This may be one approach to independently evolving mechanical abilities (structure, domain-specific mental modeling and problem-solving, best practices) separately from fundamental personal development, which may (seems to?) go a little slower.