Very curious myself to see how this does.
It looks like your on the right track with this
Full flow into both upper platform and lower fluid bed chamber.
And going with he bigger pump would be the safer route allowing more flexibility and water control.
I almost want to see a cover on the header box as well.
with as much water that you will be pumping down into the trap, I could see water splashing up over the sides as it hits the opening of the BGT.
With a cover on the Header box (removable of course), all water is focused/forced down into the BGT. bottom, sides, and top.
This I think, would help with pressuring the fluid bed portion as well as keeping a good flow over the grizzlies.
But of course wait and see what the water does first after you get the pump.
2 more cents for you

And your stand will last for ever!! That is a stout stand for sure.
Kudos to you CrisinCO