Countertop underlayment question

[Zimmy]

I've built a prep counter and bar on either side of my oven area with mortar/rebar-filled CMU structure for the base. I am putting quartzite slab as the surface (leftover from the kitchen remodel slabs). What is the appropriate underlayment material? I am trading between 1/2" cement board and 3/4 pressure-treated plywood. The cement board is rigid but not strong while the plywood is strong but not necessarily flat or as rigid. There will be steel rectangular tube supporting the overhang for the bar and I plan to bed them in with a layer of mortar or 50:50 sand/fire clay mix to level and continuously support them, then use Tapcon screws to attach the underlayment and steel tube to the CMU structure, and silicone adhesive to hold the slab in place to allow weather-related expansion/movement and seismic reinforcement. What are your thoughts on the underlayment choices? This will be exposed to the sun/rain if that affects your choice.

[Backyardchef]

Id avoid pressure treated plywood outside 'cause it can warp over time. Cement board alone is not strong enough. Best option is a flat solid base like exterior grade plywood with cement board on top or a full mortar bed over the CMU