Having a recirculating DWC with a separate rez is very good, not only does it keep the pH stable, you can control all pods from that rez.
I have 8 x 5 gallon pods and 1 x 90L rez. Air stone in each pod and an air curtain in the rez.
Every fortnight when i'm changing out the nutrients. I switch off the recirculating pump, and i use a spare pump and long garden hose to pump the old solution from my rez/pods into the toilet. I clean out the rez. I also clean out my bath and fill it with tap water, bring it down to ph5.8, add some H2O2. I then put the spare pump in the bath and pump that water from the bath into the rez. I'd fill the rez until the water is 1" above the bottom of the net pots. In half an hour i'd pump the water back out of the system and into the toilet. I'd fill the bath up again with just tap water. I'd shut off the inline valves that go from the rez to the pods and fill the rez up with the tap water. When the rez is full i add my nutrients ( for 90L of water) and pH it. I switch on the recirculating pump and pump the solution from the rez into the pods. When the rez is empty, i switch the recirculating pump off, i fill the rez with water again, add nutrients and pH it again, switch the recirulating pump back on until the nutes are flush with the bottom of the net pots. I'd fill the rez up to the same level as the pods, add the rest of the nutes, ph it and add H2O2. I open the inline valves and switch on the recirculating pump. Put the blackout lid back on the rez. In a couple of hours i'd check the pH and adjust it, if needs be.
You could add a 3 way hose valve to the recirculating pump to pump old nutes out and refill the rez by hand with a bucket. But using the spare pump, there are no drips or spills, and keeps everything clean and tidy.
You only need one recirculating pump, to pump the nutes to the pods, gravity takes care of the rest by pushed the recirculating nutes back to the rez through the pipes at the bottom of your pods.