Tile underlayment, Ditra vs wire mesh lath
"Except for a few smaller grout cracks they were relatively okay and flat"; Ditra is primarily a water blocking membrane. If you believe that the "sagging/flexing plywood" was caused by water penetration, then an upgrade to the water resistance of your assembly is in order.

However, mortar and mesh provide a function unrelated to Ditra; it is to provide a strong and flat substrate to support the tile installation.

I recommend both! Ditra to waterproof the plywood, then cement tile-board or a mortar bed to provide a strong surface to place your tile on.