This morning I ran one more simulation. I still couldn't get a nice gradual pressure change over the height of the sidewalls. So, instead I split them into three different sections. I applied 1.7 psi to the bottom, 1.1 psi to the middle, and .56 psi to the top. You can see the divided sections here.
I then ran the simulation and here is what resulted, 1800 max psi. This is roughly 1/3rd what the plywood can take so I should have plenty of safety factor.
The end result of all of this design work is that I'll be saving around $55 in materials cost. Considering my total estimated cost for the tank is now around $300 (I haven't figured out all the insulation costs yet), that is a substancial percentage of the cost of the tank.