I work in 3ds Max, and regardless of the file format, I usually just remodel the walls, floors, ceilings. They’re usually simple enough that making a new box or plane and then remodeling it to “trace” over the original walls, etc. is usually a lot faster and cleaner than me trying to salvage the imported geometry. Imported props can be trickier and usually I won’t remodel those (unless they’re small and simple) because it would takes too much time in the production cycle. If they’re imported in all triangulated and messy, then I would just have to find a similar furniture from an existing library. Those are heavy on geometry count too, but easier to clean up by removing edge loops where not needed. It can be tedious work, but there a lot of tools and other scripts out there that can help aid the process to make it less painful. That’s the general overview for my workflow. Others probably do it differently or have a better way.
Lightmap UVs depending on how clean the geometry is, and I’m usually lazy so I’ll do a flatten mapping and pack UVs. I also tend to keep the lightmap resolution down as low as possible while maintaining quality. It’s generally not good to just set something like 1024 to everything in the scene because it makes your light baking process a lot slower and also adds more to texture size usage.