Add this: instead of using just plywood, consider, since its under the washer and washers are nortorious for leaking, use marine plywood.
Then, screw the plywood down, not nails. Screws will hold it down longer and better than nails. Use a good zinc or galv screw like the ones used on decking.
Then when installing the drywall, use cementious board on the bottom one foot all around. This will stop any further osmosis or leaching.
Other than that, you have a wonderful plan.
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