Hello,
In order to find out possible replacement to OpenGL, GRX is the perfect choice. I would like to recommend it. GRX working, clean, fine, fast, ideal.
Probably GRX could be better known ... since it allows huge graphical possibilities.
Best wishes
Example of hello world on Rpi PI.
https://github.com/spartrekus/Fast-GRX-graphics-library-for-X11