I can see wanting the flexibility
And there's your answer. 2 words. Variable speed.
I keep worrying the pond pumps I have now are not going to be up to snuff since there is no way to figure out what true head pressure will be after pumping through 15'+ of piping with all kinds of turns and then up 6' to the top of the tank.
These pumps will take all unknowns out of pump choice
as far as emergency tank draining, that is being done with a central drain in the bottom of the tank, outside of the overflow boxes, that will gravity drain out at full siphon if needed.