drywalling tomorrow!
Here are the updated drawings. I separated the overflow box outlet pipes to make them more manageable.
The sump is now 2' x 2' x 6' = 180g
It is connected to two additional 30 gallon resevoirs providing about 40 gallons of additional buffering. I don't know if I'll need this, but it's available in case I do. The impedance between them is high so this won't help with the surges but may mitigate some emergency overflow events.
The overflow now has 5 separate connections. 3 go to a BA overflow (valved main, slow, emergency), 1 goes to the pump that recirculates to the surge (to tank), skimmer (to sump) and chiller (to sump), and the last 1 is an aux drain that activates when the surges do to provide additional flow to the sump.
The surges have built in overflows that flow into the main tank through the same pipes.
I have a dedicated Kalk bucket in the ATO path that overflows to replenish the sump.
Tank coming in 2 weeks or so.