Yes, Ardex is the best… expensive, but it is the best for bond strength over plywood. I was a flooring contractor for many years, and used several products, and ardex claims you don’t need any mesh along with it like the others do over plywood. The level quick rs at home depot smooths very nicely, maybe nicer than the ardex, but it requires a latex primer and mesh over plywood. (I would use a latex primer anyways, even with the ardex), just add some water and wipe it on the plywood with a sponge first
But yes, if there’s flexing, you may need mesh too.
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Sistering” and leveling floor joist would be better option if access from below is possible.