Low water flow/pressure

I know they are not the same, but here's the history. I've never had great water pressure and recently it's been getting worse, especially when more than one water tap is open, so I took the following actions. Checked water pressure on outside faucet 40-45psi. Called the water dept. to see if they were having any problems...no. Discovered part of my water line from the street was 1/2"....replaced it with 3/4" pvc....no leaks. Noticed the water pressure after the water softener increased significantly when bypassed...replaced resin. Water softener is 8 years old. I am getting pressures from 80psi with no faucets on to 20 when 2 or more are open. I do have an expansion tank on the cold water side of the water heater. We had pretty good pressure after I made the changes, but the pressure was creeping up to 80psi. I thought that wasn't good, so I adjusted the pressure down to about 50 but it still creeps up to 80 and like I said can drop to 20. Should I be setting the PRV with a water tap on or off or does that even make a difference? I can replace the PRV, if necessary. I just thought I'd get some opinions from people who know. Thanks

If the pressure moves off of what the PRV is set at the PRV is bad

Was the old city supply PVC as well?

Check the inlet screens on faucets and softner.

From the meter at the street to where it joins 3/4" copper under the house, it's all 3/4" pvc. Checked all the faucet screens, cleaned out the softener and put in new media. I'll check and see if there is some sort of screen in the softener control head. I haven't checked the screen in the pcv. When I open up a couple of faucets, the pressure just drops off more than what I think it should, especially after making the other changes.

I would have just used copper all the way around. The 3/4" plastic pipe has about the same diameter as 1/2" copper. 1" plastic pipe should have been used to equal the 3/4" copper ID inside the house.

Are there any kinks in the copper line?

No kinks...it's all sweated copper. There are pretty long runs to and from the softener. Could too much copper run in the crawlspace be part of the problem?

I appreciate any help, BUT the diameter of 3/4" is NOT about the same as 1/2copper pipe.

Plastic pipe has much thicker walls than the equivalent sized copper, so, yes, a piece of 3/4" plastic pipe's ID
is closer to that of 1/2" copper. Now, waste pipes are measured by the ID, but most supply pipes are measured by their OD (that's why, for example, a connector like a Sharkbite works on pex, copper, and cpvc - the OD is the same on all).

The area of 3/4" ID PVC is almost twice that of 1/2" copper ID. I can visually see that. If you like, I could calculate the difference. Like I said, I appreciate people trying help, but misinformation doesn't help me or anybody else.

3/4 pvc or 3/4 cpvc there is a difference and pvc an not be used for water distribution piping within a structure.