That is looking great Chucker! Not being a woodworker anything I add would probably sound dumb
I am practically skilless at this kind of thing. Something I would look at is adding either a 2X2 center brace in the bottom or 2 smaller ones across because once again sagging. With all the water weight and weight of soil you would have alot of weight on the bottom plywood and now it is off the ground a few inches. I would expect some sagging in the middle. My husband built me some 18 and 10 gallon tainers and they are very heavy. Have you thought about putting casters on the bottom to help with moving them?
Kat