Since you're on septic, you might consider where exactly all that water
will
go when it comes time to drain it. As a general consideration I don't put
any water near or around my tank, drain field, or any locations upstream.

Good point.

"Around here" new installations have interceptors above the actual drain
field. The interceptors take water dumped above the drain field and take
it below the drain field. But usually the drain field isn't immediately
adjacent to the tank. In any case, he can run his pool draining hose to
below his septic field.

But I live in the PNW, watering the lawn is rarely required.

Remember the 70s book, "The Grass is Always Greener over the Septic Field?"