hey @nouvmap - other people would probably do a better job / use different software but I hope I've got the gist of your question right:
I think you are talking about going from a texture like this (exported photogrammetry software):
to something more like this:
I admit this isn't a perfect example (there's still a lot of tiny UV islands around the edge of the second texture map) but you can see the second texture seems more organised to a human eye.
I achieved this by loading my mesh into Blender, duplicating the mesh, reducing the face count of the duplicate, UV unwrapping the simplified mesh, then baking the texture from the high res mesh to the smplified and saving the result.