So I finally had a cool enough night to continue with my seasoning inside rather than on the grill. Used the shortening as suggested, but mixed in a bit of sunflower oil as well.
First couple times, at 550° for 20 minutes resulted in some nicely browned color as shown in the first two pics. The next night I tried it again, same approach, but as a result ended up with this cracked texture as shown in the last two pics. At first I thought it was flaking off, but it's not. Just a cracked texture.
Any idea why it happened our how to resolve? Since it's not flaking off I'm thinking I could just apply another coat and bake again, but not sure if I should bake it lower and slower, or take some other approach.