McP77

It depends on the adhesive you are using. Fastflex is designed to go straight onto floorboards. It will work without plywood.

Something like your single part flexible is designed to go onto a plywood overlay of 15mm, 18mm in practice.

With your 6mm and 12mm plywood situation you are in between the two - overkill for Fastflex, not really enough for single part flexible. Even when using something like gold star with flexible additive (more flexible than single part) the data sheet still specifies 15mm ply.