Hey, concept artist for the project. Without baking lighting it looks great, no problem at all. We started with a 64bit lightmap and have since upped it to a 1k lightmap. The picture above is of the 1k map.
It looks great without lighting build (nothing up with the normals or smoothing groups), just goes sour with baked lighting regardless of build quality.
Your lightmap UV’s are not very good, you split it up far too much
Also, you have a bunch of intersecting pieces, which will cause issues because the areas of the polygons that are hidden will bleed shadow over onto visible areas. Also, the small parts are smaller than a pixel so they will not look good. And, you need more space between UV islands.
After fixing your UV’s you’d need to adjust your lightmap resolution