I would use the middle shelf on only half the unit, and leave the other bay larger, more open for canister filter, CO2 or other large things.
2 x 4 is plenty strong enough. No need to go to 4 x 4 in the corners.
X across the back, or even a sheet of plywood as a shear panel would be a very good idea.
I also would not use the 2 x 6 top, 3/4" plywood would be very strong, or add perhaps 1/2" or thinner plywood on top of the 2 x 6 for a more uniform surface. Even a sheet of Styrofoam might be enough if the top was well made, very uniform.
Most of my stands are 2 x 4 on edge, boxes that exactly fit the bottom rim of the tank. Most of my tanks are glass tanks designed to sit on their rim.
Sit those boxes on concrete blocks. Add 1/2" Styrofoam to even out any irregularities in the wood.
My acrylic tanks are fully supported. I used manufactured stands and sheets of Styrofoam for them.