I don't understand your test and I don't see a problem. The CSV is maintaining 50 PSI constant as it should. The 120 PSI back pressure just means you could open another faucet or two and still have 50 PSI constant as the back pressure comes down to signify more usage. All you are testing is how much water can go through the hose bib at different pressures. At 50 PSI constant you can only get 5 GPM through the hose bib. Without the CSV the pressure is higher and you get more flow through the hose bibb. But if you restricted the hose bibb to 5 GPM as before, the pump would cycle on and off without having the CSV.
Re-reading back to 2023, I don't think there was a problem there either. When running two sprinklers the pressure dropped to 32 PSI, and I don't see how the back pressure could have been 120 PSI. That is because when you open enough hose bibs to get the pressure below 50 PSI, the CSV just turns into a piece of pipe. At 32 PSI the back pressure should have been about 40 PSI because you were using all the water a 3/4HP, 12 GPM pump could supply.
If when you open enough hose bibs to bring the pressure below 50 PSI, the back pressure on the CSV should drop to almost the same. As long as the CSV is holding 50 PSI as it should, it is doing its job.
Now if you want more pressure the CSV125 also comes in 60 PSI, but I think you are out of pump and need a larger one for what you are doing.