Update:
I’ve been trying a lot of things and gone down a lot of rabbit holes, none of which have really led me anywhere.
In a “i give up” move, I tried opening the substance in substance player and exporting bitmaps as jpegs along Diffuse:
Glossiness:
Height:
Mask:
Normal:
Specular:
I then uploaded the individual “bitmaps” (?) as textures in the corresponding materials menu in SketchFab:
The model is produced is this:
The size of the bricks is all wrong of course and if you look closely there’s repetitions in the overall texture (which defeats the entire purpose of creating the endless texture, right?), but I guess it’s the closest I’ve gotten to what I’m going for so far.