If pressure is good but flow is not sufficient it means that you have a high enough static pressure but residual pressure is not there. That hints as a restriction somewhere upstream of where the drop is measured. Every time your water changes direction or is squeezed through a valve or other device it "loses" pressure. If you have a backflow preventer put a gauge on the first port and see what happens there when you have a heavy flow.
Take a look at your nearest fire hydrant and see what color the dome is painted. If they adhere to national standards the color will give you a hint as to the quality of the water supply available in the area. If its blue (>1500gpm)or green (1000-1500gpm)you should have good water supply, orange is marginal (500-1000gpm)and red (<500gpm)is not good. If the nearest hydrant has a red dome your problem may be with the availability of water supply in the area itself, and a new line from your building to the main may not fix your problem. In my past life as a firefighter I've seen residential hydrants that would not flow 100 GPM, and they are hooked directly to the water main.
In the past I've managed to get by with substandard water supply by gating all rinse tank valves way down. My water wizard pump uses 35GPM, but I can gate the valve down to 10 GPM and it has plenty of time to catch up and refill the tank during low pressure wax, SFR, and dry before the next car enters. You can also gate down your SS rinse tank to around 12 GPM and it will be fine for a 3 bay. As you likely know most float valves are trying to flow in excess of 50GPM but there's no real need for that high of a flow. Even with one auto and 3 SS bays and SFR production you could get probably by with a total main flow of 40 GPM flow rate if you had to.
Preferably with everything running your incoming pressure should get no lower than 40 psi. Hydrominders are rated for 40psi inlet pressure but will work at 25psi (per a hydro systems engineer that I talked to many years ago). Ironically a hydro at 25psi is going to put out a stronger solution than a hydrominder at 40psi(again per hydro) . The important part is the pressure to all hydrominders and injectors is the pressure is consistent so that the ratios stay the same.
Also all this problem could be as simple as replacing a badly corroded incoming valve or occluded galvanized plumbing , so look at that before you spend >10K on a new line. I recently replaced a 2" galvanized nipple on a main that was close to a copper line and even though it was only about ten years old it had a hunk of corrosion the size of a large marble in it.
