How to merge surfaces that lie in the same plane? I guess it's like undoing a subdivision, except it's not always neatly subdivided into even quads. How can you merge a surface made up of multiple triangles and quads/gons into a single one?

Since I'm dealing with a 500mb obj file I'd really appreciate if there's some fast way to do it for the entire model automatically.

Edit: I tried googling "merge coplanar faces" and I got the suggestion to do a "Limited dissolve (object mode: press x, then limited disolve) but this crashes blender. Any other ideas maybe?

I'm running 16gb ram, ryzen 5, nvidia tu116
There is the Decimate Modifier, with it's Un-Subdivide option.
Very useful for undoing unwanted subdivisions.

Set the number of iterations as needed.
Use 2 iterations to undo 1 subdivision step.



To better see the end result, you can enable wireframe overlay in the 3D viewport.