not saying a pyrimid shape just a wider base and skinnier top could help stability butwould result in less water pressure But youre not going to get much pressure at 4/10’s pounds per verticle foot
Stainless ? who can afford thatstuff any more
polly is good see H.D.P.E products used extensively in construction
Actually you do not lose any water pressure because of the shape of the tank.
The only thing that matters is the height of the water column.
You will lose water pressure more quickly however as you use water because the tank level will lower more quickly.
I do like the idea of burying part of the tank. You could probably get by with only a foot of concrete for the footer by doing that. Of course you would have to add a pump to the system to get access to the lower level water.
No telling what it would take to get a construction permit of something like this.