I am on "city water" and do not have a PRV on my line (as far as I can tell.)
I do have a water softener (since our city water is still pretty hard.)
When I measure "static" water pressure in the shower at the highest point in the house - with nothing on - I measure 75psi.
Then - when I turn on a faucet in the same area - the pressure will maintain around 65psi.
When I turn on the clothes washing machine - the pressure will drop down to 30psi.
But - when I flush the toilet (older Kohler Rochelle "silent" toilets) the water pressure will drop down to 20psi or in some cases as low at 15psi.
Is there anything I can do to maintain higher water pressure in the shower when the toilets are flushed?
I would rather not replace the toilets... but if there's some kind of flow / pressure restricting device I could put on them... I would do it.
The first thing to do is operate the faucets so you only have the 30 psi, then push the BYPASS button on the softener and see if the pressure goes back to normal. If it does, and since you do not seem to have a pressure regulator it should, then you have a plugged water sofener.