Laird cross

I’m pulling my hair out trying to figure this out. I installed a heatilator el42 wood burning firebox. It is 8” off of the floor of a raised foundation, advantech subfloor. I plan to cover the sheet metal all the way around, making the total height of the hearth extension 14 3/4” of of the subfloor. I have a 10’ break and made Z flashing out of 16 ga galvanized to cover the bottom framing and protruding onto the subfloor 8”. My plan was to install cement board over this, set in thinset, then screwed down. On top of that metal framing then full brick. The hearth extension will end up at almost 15” tall, extending out 20” from the face of the firebox. Do I need micore instead of the Durarock as a base on top of the advantech subfloor? I’m a commercial builder and this is my first rodeo with a fireplace. This is my own home and4 years into the making. I’ve built all from the ground up. I just want it right.