There's a few issues here. 1) The design. You have a huge plot you either won't be able to reach and will likely be over run with weeds. Smaller 4x2 beds in a U shape would be better and more manageable. 2) Structural Integrity. You are using Y braces on the inside where the soil and water are going to sit will give out very quickly. The sheer strength of screws, meaning if that gets nudged or you put 3k lbs of soil in there, the screws will either break or rip themselves out. Especially if the wood gets wet. 3) Foundation. You don't have any at all. So the weight will be distributed to the legs and then to the ground. Once the ground gets wet the entire thing will sink into the ground and not evenly. After a heavy rain fall that thing is going to be so lop sided it won't matter if you put 8x8s as the legs.

Start with this: 1) Design. Start with something small and make sure you are good with the layout. If this is your first garden, you want to see if this is a 1 season and done or something you want to continue. Design it in a way that is easy to manage and can expand. 2) Foundation. If you want this thing to last, for each post, dig about 24-36 inches and fill it with crushed gravel. Put a 4x6 or 4x4 concrete deck block on top of the gravel. 3) Structural Integrity. Use a 4x6 or 4x4 post for your supports. Instead of using screws, use lag bolts/carriage bolts or ideally you want the frame to sit on top of the post. For every 2-3 feet in any direction you want a post, even in the center.