Building with wood?
pls redesign this platform so that you have solid wood support under the washing machine -- all those gallons of water make it heavy. A couple of 4x4s with eight inches in between should do the trick. then standard 5/8 inch plywood covered with a hard floor covering material [so the washer's feet don't gouge the plywood] should do the trick. btw, you also have to have access to connect the hoses and power cord when doing this -- the usual way is reaching over the top ... you might look into a means of making the cabinets over the machine easily removable while still being strong and solid.
3/4 Baltic birch would work great if you plan on finishing it. You’ll want support under the plywood to prevent it from sagging when the washing machine is full of water.