Water Pressure question and pipe sizing
300 feet of 3/4 poly is, itself, a restriction. That's a very long run for pipe that small.
Adding a section of 1/2 PVC doesn't help, but unless it's quite a long section, the 300 feet of 3/4 dominates the total dynamic head (which is what limits your flow.)
Is your incoming pressure so high that it would burst the pipe? If not, skipping the path through the PRV and house might be the easiest solution, as there would be more pressure head to start with, and you can put a garden hose or irrigation pressure reducer (quite inexpensive) at the end of the line, if needed.