From what I understand about denim, when it is washed/dried it does not shrink uniformly as it's made from different size yarns. Now this is just an idea but maybe running it through a dryer on no heat with a few dryer balls or tennis balls might do the trick to soften up the fabric a bit. I remember hearing something like this done with the old denim Utilikilts to soften them a bit. Hopefully someone with a bit more experience with denim kilts will be along soon...