It will work.
I just recently did a similar concept. But with a twist. I do 3 tanks. One water change flows into the next one, and last tank dumps into drain.
Yes you do have to turn off ATO during drain time. Yes you need to ensure return pump level doesn't go too low. I manage to do 1.5 gallons daily with no issues. (**I did raise ATO Sensor Level to give me more volume **)
I did play with float switches to see if I can extract exactly the same amount I add. That turned out to be very tricky alignment. The pumps I used are accurate enough.
So instead I use the float switch in each draw/return area to enable a safe go ahead on the water change. The float switch must be up which prove there is enough water in return section to not run dry during extract.
It has been working for me for 3 months. I am re-using salt water (SPS tank gets new mix, next is lightly stocked frag tank, and dirty lps/mixed tank get the last water change).
My new salt water reservoir is a old 65 gallon tank I tried to sell but nobody made a offer. That gives me worry free 2+ months of daily 1.5 water changes.
Hard to film all this working on the Setup, but this demo I did before installing the Controller givex you the idea on how things work.
The Blue Flask (Left) is the New Salt Water Reservoir.
The Blue Bottles (Middle) are the 3 Tanks Sumps.
The Empty Flask (Far Right) is the Basement Floor Drain.
The pumps I use are these [KAMOER Peristaltic Irrigation pumps] (They do go on sale for $27.99 each). Fast, quiet, long life, and can out-run a DOS Stepper-motor-Type without any over-heat or life reducing issues. (Can run full speed for an hour if needed).
Smarter Shopping, Better Living! Aliexpress.com
www.aliexpress.com
You could run them off you Apex Power bar if you have two spare outlets (and a AC adapter).
Nothing wrong with a Neptune DOS pump. I actually think it's a beautiful, powerful, accurate Stepper Motor pump, which is best used for accurate, reliable, life long dosing, not pumping large volumes of water continuously. I heard that the Neptune DOS WC Programming logic... breaks up the Water Changes into small repeat cycles to keep them from burning out.