Hey decod-fp,
Apologies for not getting to your post sooner. Sometimes issues fall through the cracks, and someone is never assigned. I will do what I can to assist you.
So firstly, reset all of your ‘World Settings’ for Lightmass back to their default settings, and be sure your ‘Lighting Quality’ is set to ‘Production’
When you import your mesh, if you have your own UV’s set up, be sure the spacing is even and it is not too crowded. You mentioned to even tried to create multiple UV maps for your mesh, which is sometimes necessary. You might want to try combining that and increasing the resolution to 1024 up from 512.
Let me know if these suggestions worked, and provide me with some screenshots of your results, including the UV’s of your Static Mesh please.
Thank you,