I took a really awful* graphics programming course during my Masters degree, to the point where I didn't drop, stayed in, and took the bad grade just to leave a final review about how bad it was. I later discovered processing and graphics programming was a breeze. That is really what the course should have been taught in. I've used it multiple times since to make animated graphics for videos, 3d text that I couldn't figure out how to make in Photoshop or Gimp, and a visualization for the cue ball's departure angle in billiards.
*I believe my issue was it never built up the mental model of OpenGL to the point that I couldn't comprehend the difference between a VAO and VBO. [1]
Ouch, sorry to hear that. Oftentimes university classes are more about giving a theoretical background instead of hands on practice. The theory is useful, especially if one is trying to write performant code, but it's a big wall to scale before being able to play.
Not everything needs to be optimized to the maximum these days, with computers being so fast, and oftentimes inefficient code is fine for just playing around. In your example of graphics APIs, the barrier to entry has gotten much higher with programmable GPUs if one wants to use APIs such as OpenGL, D3D, or Vulkan. It used to be that one could just draw polygons on the screen with C code that would fit in a single page using GLFW, OpenGL immediate mode, and the fixed function pipeline.
Hilariously enough, you can see the difference between the deprecated immediate mode API (8 function calls) and the "recommended" way on this Stack overflow answer (fragment and vertex shaders, a few buffers, and way more code): https://stackoverflow.com/questions/6733934/what-does-immedi...
On the other hand, if all you want is to draw things for fun and you're not trying to write a AAA video game, a scene graph-style API or interactive environments like processing are much better fits, as you point out.
*I believe my issue was it never built up the mental model of OpenGL to the point that I couldn't comprehend the difference between a VAO and VBO. [1]
1. https://www.khronos.org/opengl/wiki/Tutorial2:_VAOs,_VBOs,_V...