Consistent error when importing decimated fbx from zbrush

Hi everyone, thanks so much for your help. Relative substance newbie here.

I'm trying to import .fbx files that I've made in Zbrush, into substance.

I'm using a macbook pro.

The files are characters I've made. Each one started as a few million polys and I've decimated them to a max of 250k polys for the character and 20k for each of their smaller bits of clothing.

99% of the time when I try to import to substance, I get the following error when importing, then the import gets stuck on around 7% and either crashes or takes way too long, so I have to cancel it.

Error: [Scene 3D] [FBX] : Mesh normals are invalid (some values are null) and will be recomputed.

I have uploaded screengrabs of my export settings from zbrush and import settings to substance.

Please help! I'm going to have to resort to painting them in zbrush otherwise which will be a super frustrating experience.

Do I need to tweak my settings? I've tried uploading to Blender and re-exporting. No joy.

Thanks so much. Praying someone can help me.