Hello ,
Thank you for posting in the community and it looks like there is not an option to scale the roughness map independently from the default texture. I have not found a way to extract the maps from the default materials and not sure if we can provide them as they could be licensed a certain way. Have you tried to apply as decal and see if that was giving you better results?
You can make a request for an idea on more scaling options for maps on our road-map:
https://portal.productboard.com/7pu88c9kpmqtzt8hwg6arujh/tabs/4-under-consideration
Kind regards,
Vincent B.