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.