FWIW I've redone a number of washes to eliminate the GinSan timers and pushbuttons and replaced it with rotary switch and Dixmor timers. I find it to be very easy, although I've been doing this for 36 years and have worked on some 500 different washes.
You can run multiple bays with one large transformer, but I don't like to. If that one transformer fails, you're completely down until you can get another, and they're not easy to find. With a 100Va transformer per bay you can scavenge off the next bay temporarily if you have to, and just about every car wash supply place carries them.
95% of washes I've worked on have no need for the timed load to go back to the room, especially with a Dixmor timer since you can enable washdown in the bay with the remote. To me it's just complicating the wiring for something you're unlikely to use.
When you have a common pump for multiple bays, there's a controller that turns on the motor. You do use one stack for the high pressure pump motor and the other just opens solenoids.
