Am I reading this wrong but are you talking about an overflow to get rid of filtered
water. Sound like a bit of a waste to me.
Get a plug in electrical timer then plug the pump into it.
Work out how long it takes to fill the tank set the timer and then the pump will stop before the tank overflows.
That's true, but an r/o takes longer to process water the colder the water gets. I tried to process water into my trailer over night and often either overfilled it or underfilled it.
The overflow is just in case he forgets to switch the water off. A flooded shed or garage takes ages to dry out. It's a good safety precaution.
Our overflow on our tank hasn't been needed once in 6 years. We have a float valve in the tank that operates an on/off solenoid valve. But its there if something goes wrong.