He has scads of money now. I would think that he can afford to buy proper carving tools, instead of using his hunting knife.
I'm also really bugged by the way he holds that knife for incising into the magic stone. He really should be holding up way close to the tip. Perhaps that is pictured in this way so some of the younger readers are not encouraged to hold knives in an unsafe manner.
Being an isekai story, he needs to invent the Magic Dremel Tool.