With your pressure dropping so much i wonder if there aren't too many sprinkler zones open or too many sprinklers on the zone you're trying to water. It also suggests you might have a leak in the sprinkler line pouring water into the ground unseen.

I don't have a well but we have pressurized water systems from a spring that include a pump and pressure tank. Do you have a pressure tank on the system to boost pressure? Your faucet pressure is pretty high at 53-65 lbs., enough to cause more problems with faucet seals in time.

I would be concerned about water safety given the sulfur smell. Do you use an ultraviolet sterilizer? A water test would be a good idea.