Water supply slowly loses half pressure
A suggestion to keep a pressure reducing valve from causing low flow is how I see this as a question.
The problem sounds like your valve is rated for a lower flow rate than you want.
An example of this you could install a 1/4” valve with no flow you measure 60-70 psi just as you do now. Opening the valve relives the pressure on the system and once any tanks like your water heater and the plumbing are equalized you have the flow at the maximum rate the valve can supply.
With a 1/4” orifice the valve is restricting the flow thus the pressure drops I have seen this with city water systems where the owner just grabbed a 1” PRV and did not realize it had 1/4” orifices and their sprinklers would not work on the low flow. I replaced the valve with a PRV rated at 20 gallons a minute  (the city max delivery rate) and the problem was solved.
So your valve internal orifices are two small in my case I actually had to get an 1-1/4 PRV to get the 20 GPM valve it was a bit more expensive but they never saw the drastic drop in water pressure after that and they used 60 psi as that was the max recommendation on some of the components in their system they said it worked better than it used to probably because the original valve had smaller orifices than the valve I used therefore more restrictions.