I agree that sdl not getting a simplistic high level 3d api is a missed opportunity. The current 2d one can already be use for 3d, albeit it comes with innefciences ( see Am I using SDL_RenderGeometry correctly? - #3 by Sylvain_Becker )
So better 3d support would not have meant bloating sdl3
As an aside, could you share your 3d api built with RenderGeometry?