I don’t really know, but I think using full precision UVs on one mesh doesn’t really matter. I have it also, and haven’t noticed anything unusual going on.
Changes to the lightmap will have no effect, as it is the UVs that describe how the texture is laid out.
It also doesn’t matter that it’s a huge mesh, because the UV info only relates to a 1 by 1 square which is repeated everywhere.
If you’re really worried about it, use a smaller mesh and make your material UVs world position. This si something done in your material, not your modelling software. It means it’s no effort to lines things up, because the UVs aren’t on your model:
