That sounds like a fun challenge. If you're constraining yourself to use as few ...

That sounds like a fun challenge. If you're constraining yourself to use as few libraries as possible, I'd go with OBJ [1] for the 3d mesh and PPM [2] for writing images. It's easy to implement a bare bones reader/writer and some OSes (like macOS) can show them in the file browser. Raytracing in One Weekend goes over PPM. There are a bunch of header-only libraries that handle different file formats like stb_image [3]. I usually end up using those when I start dealing with textures or UI elements. I don't use Windows so I haven't used their APIs for projects like this. I'd usually go for imgui or SDL (like you mentioned). tinyracaster, a sibling project of tinyrenderer, touches on those [4]. I liked LazyFoo's SDL tutorial [5]. Good luck!