One other thing. Your plywood is cut wrong. The grain should run top to bottom not front to back. And it's not just cosmetic. Although plywood has multiple plys it is stronger in one direction. That direction happens to be with the grain, just like solid wood. As has been said: Glue is your best friend.
BTW if you had used 3/4" plywood and proper joinery the 1 x 4s would not have been need for anything except trim and you wouldn't have needed a single screw.
You're putting a 29 in front. And a sump inside. How will you access the sump?
If there is a complete frame of the 1x4s on the inside, the plywood will effectively just be cosmetic, with some reinforcement against racking. But, yeah, you can do a stand with just plywood, and have a quite strong one at that. I don't have the precision to cut stuff that accurately my self, but for those who do, they can build a simpler stand with fewer materials, and have it come out quite strong and nice looking.