Hi Gary, nice schematic....
Most only consider the water volume 'in the fish tank'.
I was told that the rule of thumb about moving the water twice an hour....came from 'aquaculture'.
Considering a 'balanced AP system'....I've read where that much flow, isn't necessary. Some successfully go with less then 1 times an hour. They are using air pumps in the fish tank too.

You drew drains on every thing.
In order to save on costs, I didn't add any to my system. If I were to it again, I 'might' put one on the FT. The grow beds can be drained by just removing the stand pipe from the bell siphon. My sump(s) are in the ground...FL is flat, no where for it to drain to... ...
I do have a nice sump pump, that has threads for a garden hose.

One thing you might consider....is a 'high point' overflow(s) on your fish tank. Mine go straight back to the sumps.
If work needs to be done on the GB's, the water flow into the fish tank can still be maintained. When it's really cold, the GB's can also be by-passed, since that's an area where a lot of heat is lost.