Advice? Macarons top is hard and crumbles when cut in half

I made a post earlier about my third macarons attempt. I am struggling with the tops/shell getting overly hard and then crumbling into a bunch of pieces when bitten or cut. She’ll is full and not hollow, just hard top. Could it be from overcooking? I did 18 minutes at 300 degrees as the recipe stated but now I’m seeing some people say 15.