If your tank is larger, the lag time between the respone time for the solnoid will always have higher CO2 for the return water than what is in the tank itself.
The plants use up the CO2 and then you have CO2 removed some from over flow.
Rather than having the pH probve on the return line, having in the tank where the CO2 would be the lowest would be a better loctation.
Where you have it now, the CO2 will be the highest.
Response time will higher if you move more water through the tank,have better circulation patterns/mixing in the tank.
The other thing you can do to improve the reactor, feed then CO2 into the lower intake side of the pump loop, rather than after the pump.
This will improve the dissolving power and capacity for larger tanks/systems.
You'd basically drill into the side of PCV near the bottom adjacent to the pump inlet. Add a piece of airline to feed the CO2 into the suction side of the pump inside the main PVC tube.
You have backpressure since the input form CO2 is on a pressurized pipe of smaller diameter. So if the gas CO2 tank runs out, it'll backflow easily.
Your solenoid will also not close using a pH controller because the controller will "think" you need more CO2 gas and will leave the solenoid "open".
On the lower pressure suction, the suction will pull the gas out of the tube and there's is less backpressure inside the larger diameter tube as well.
And it's more efficient location to start dissolving the CO2.
This is how my CO2 reactors are set up, I use a sump so it's not a closed loop, but it's the same design.
Making your own CO2 reactor is not rocket science, and yes, I've seen a number of CO2 reactor knock off designs of mind over the years.
The way to combat those companies from stealing your design: tell everyone on the web how to DIY build your own for peanuts, so if you can make one for 2-10$, why pay 50-80$?