What a fascinating series of posts. I've been thinking about GUI toolkits on Kobo's for a while now, investigating different strategies.
Ideally, something like
littlevgl,
Dear Imgui, or
nuklear could be quite nice to use, although they all have their pros and cons.
Littlevgl looks easily portable to different devices, but its event handling seems to be tied to a refresh rate/polling mechanism.
Dear Imgui looks nice, but it's C++ and designed for OpenGL, although there is a software rasterizer that has been developed for it.
Nuklear also looks quite nice. It too is OpenGL, with a software rasterizer that has been written. It's a single header library, which makes for easy deployment. Main issue is the font handling seems very convoluted to get started with.
Another thing I've been mulling is the idea of making my own toolkit, very much like KSSM here (although probably in Go).