I looked at the 1800 specs and they work from 15-30psi with 30 optimum and they are designed to balance themselves to average that. It is recommended you not exceed 70psi at the inlet but the point is pressure is not your major problem.
Now then, the flow rate of each full circle pattern sprinkler is 1.8gpm at 15psi and 2.6gpm at the optimal 30psi. You said you had 5 or so sprinklers on each irrigation circuit so your system is demanding 13gpm. That is more per circuit than most residential water supplies have available so switching pipe size is not going to help. You will need to re-arrange your sprinklers to have smaller circuits or switch the heads to something that draws less water. Obviously if you have hidden pipe breaks and leaking valves you are losing waterflow too. And if you have water flow blocked by deposits you lose more.
Once fixed, I would set the sprinklers to come on early in the morning before anybody is up and taking showers. It is a good time to water for the health of the lawn and you will not have competitions for water flow.
You can call the water company and they will tell you approximately how many gallons per minute they are pumping to your house. Or with a five gallon (or a gallon one) bucket and a stop watch you can come close yourself. Just turn off all the faucets and sprinklers. Put the bucket under a hose bib and crank the sucker full on until the bucket is full. Do some math to extend everything to 60 seconds. Here is an article that discusses the method.
One thing you mentioned is that you have 115psi at the other house. That is excessive and puts a strain on residential plumbing systems. I would think about installing a pressure regulator to bring that down. 88-90psi is not so unusual but even that is high. Again the 1800s just as an example recommend a max of 70psi at the inlet.