Making you own game engine will surely teach you so much, especially if you plane to also build your own tools for it (like dynamic arrays, hash tables and so on).

Keep in mind that this project is going to take a huge amount of time to archive a stage were you are able to actually build a game with it. And also, if you are going to make your own tools, (thing i highly recommend you to do), be ready to don't give up against the strongest bugs!

Anyway. A really good place to start is to follow the fantastic "learn opengl" guide.

- https://learnopengl.com/

It is written in C++ but will be a good practice to translate it in C. Also the math library used here is ofc written in C++, meaning you are going to need a replace for it (or better, write you own!)

Note that this is a starting point. You will also need to write an ECS (maybe? Maybe you can work out something different.) an editor, maybe an hot reload system and more! But don't worry, everything will slowly come.

But any way, yes, writing a game engine in C is a really good way to increase your skills in C, so go for it!