We had a tiler do our bathroom floor. He put 12mm ply onto our floorboards with lots of screws. He put the 450x450 ceramic tiles into flexible adhesive and flex. grout. We were v. pleased with the job. So we said he could do our kitchen diner.
This room is much bigger at 24m2 and has a mixed base, part is concrete floorboards and part is wooden floorboards. We know the floor is not level all over.
The tiler's solution is to plywood the lot rather than screed part and ply part. He said otherwise it would crack where they join. He used 12mm ply with lots of screws in the wood bit and far less in the concrete floor.
He said he wanted a large tile specifying 600x600 and it must be porcelain as it needs to be stronger. He will use flexible adhesive and grout as he did in the bathroom.
Once he ply fitted we wanted elect. underfloor heating. No problem he said.
BUT I've read it should have insulation underneath. Does the plywood count or suffice as insulation or should we have had some foam boards underneath?
Will the floor be strong enough to resist cracking tiles or grout?

might be an idea to introduce yourself first sir/ma'am