So i had a pretty good idea how you could integrate this smoothly into Medieval overhaul. You can tie the Crafting of the Grace into the Alchemy Research tree.

Another suggestion is to add a mechanic for them to be able to Pray to restore a small portion of grace naturally much like meditation does for Psi powers. This would allow Nephilim to still be viable to have around if you do not have the means to make Grace yet. It is not Ideal but it would be enough to prevent them from suffering.