Off-topic, is there any mid-level 3D API that I can plug vertices into directly? Everything that I see is either super-low level (Vulcan, OpenGL) or super-high level (Unity).

Shameless plug, check out sokol_gfx.h (and maybe sokol_gl.h, which is a simple GL 1.x style API on top of sokol_gfx.h):

https://github.com/floooh/sokol