Thanks for the great post, Mikael.
OSB would have been an option if it weren't for the fact that I've already insulated, plasticed, and drywalled the ceiling.

But I realized afterwards when I was talking about the fire rating on the wall that I also needed it in the ceiling. So the only option left is to add another layer of drywall.
I used a drywall lift for the first layer. It cost 200 per day to rent at Bauhaus. Well-invested money!