Your attempt to bring up the overall co2 concentration in the large tank with sump, I have a different approach.
First of all, it is probably just me with this idea, it is not tested and don't want to lead to any problem if anyone copy it, so correct me if I am wrong.
I have a 160G display with 20G and 50G sump, the actual water volume is about 200G-220G.
Years ago when I started the CO2, I found it was an impossible task to bring the co2 to 30ppm in every part of the tank.
Since then I adapt a method by using the return pipe pushing a high saturated co2 jet stream at certain angle towards the center of the tank where most of the plants are concentrated. The jet stream creates a high co2 saturated current blow over the plants then slows down, dissipated, and mix with rest of the tank water.
I don't aim to increase the ppm(or bring down the PH) to a certain level in the whole tank, only this jet stream carry high ppm of co2.
Most of the plants are constantly enclosed by high co2 current, (the co2 concentration can be much higher than 30ppm when hitting the plants), as long as most other part of the water body is 30ppm or less, fish have no trouble, they get in and out of the current.
it is difficult to measure the exact co2 concentration in certain points of the current, and the value is dynamic, but don't care....
The benefit is:
Even the CO2 is cheap, but don't need to refill a 15lb co2 tank every month or two, ....
similar to spherical light intensity chart