How to create correct lightmap UV for complex model?

I downloaded from web model of bath. This model has many polygons and rounded edges. I have created 2 chanels for UV. First for texture, second for lightmap. For this I use 3ds max and modifer: unwrap UVW -> mapping -> flatten mapping. But every time this model get texture artifacts after build project.
I’d like to understand is there any tools for automatic creating correct lightmap UV for models like this bath? Could you explain how to use it? You can download model of bath from here bath2.FBX - Google Drive

Link to a post i made that should help

I’d unwrap it like this, but that topology is not helping.

I redid the topology and UV’s.