I don’t know how feasible for you to use an immediate mode GUI library but imgui came to my mind as soon as i read the post. However it’s written in C++ instead of C.

I never tried the C bindings but it seems to have a couple of options including cimgui to use imgui in a C project.

Maybe it’s worth a shot if you want something that’s proven to be lightweight and battle tested (I mean the main imgui project for this).