It would be an expensive solution, but consider installing an "on demand" in-line hot water heater that services only the shower.
We live in Florida with our own water well supplying a water softener with aerator tank and a 40 gallon electric water heater. This arrangement services two people living here. There is no problem with lack of pressure when someone flushes a toilet, starts the dishwasher or the clothes washer, or runs water elsewhere in the house while I am taking a shower. My problem is I like to take extended "Hollywood" type hot showers that last at least a half hour. The shower temperature is controlled by rotating a handle from off through cold to hot positions with no control of the volume of water. With a 40 gallon hot water heater, I run out of hot water within thirty minutes. I plan to install a flow-control at the shower head, but do anticipate that I need either more than 40 gallons of hot water storage or storage at a higher temperature. High temperature storage of hot water is terribly inefficient and wasteful of energy, but that is what we did in Dayton, OH with a natural gas-fired 55 gallon water heater. That puppy NEVER ran out of hot water, but you had to be careful not to scald yourself washing your hands at the sink!
I have read that with an in-line hot-water on-demand water heater, this cannot happen. Water is delivered at just the right temperature and hot water never runs out. I know for sure that our well and water softener aerator tank can "keep up" with a continuous flow of several gallons per minute, so I may try this solution when it is almost time to replace the existing water heater... which is fast approaching the end of its projected life time.