Don't feel bad about not seeing the issue sooner KP. Feel happy your still making progress and can move forward with the system you've created.
It's all about accuracy within a range. looking at the valve your using I'm guessing it's rated at probably 3,500 - 4,000psi? Maybe 2,500?
When trying to maintain 275 what's not working properly is the cracking pressure of the relief. It's supposed to hold closed and stay closed and then snap open when opening pressure is reached. When it's turned way down out of that " proper cracking pressure" range it opens slightly and slowly and in essence becomes a flow control and not a pressure relief if you know what I mean.
Question is, how to remedy? I wonder if a lower rated relief valve is available?
I think best thing would be to make your own relief externally or purchase a RC hydraulics type relief? The 4200 relief comes to mind but I know there are much better versions out there that might work ?