Many of the thinsets avertised as "flex" are simply modified with an acrylic latex modifier.
You can buy modified thinset, or you can buy plain old thinset and add a latex modifier. It replaces part of the water when you mix. Latex modifiers are sold in 1-5 gallon jugs at home centers.
I'd still use CB as a break between ply subfloor and the tile.
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