18"X18" tiles onto 3/4" P/W on 2"X10" 24" O/S

Our bonus room above garage, is going to have a small full bathroom. I was thinking we need to lay down another layer of 5/8" or so plywood before tile, would this be needed? Also can I install a layer of Merkrete 626 primer instead of 1/4" hardi CBU? I would like to keep the height down to match carpet in the rest of bonus room. I have been using Flexbond for thinset. Tiles were a closeout at Daltile they are 5/16" thick. The box trusses above garage/at bonus room are 2'X10" on 24" centers.

For 24" spacing you need 2 layers of wood, top being 1/2" or better ply.

Then I would install Ditra.