Regulars here know that my wash regimen is pretty much all about using the foamgun. Note that I don't put a lot of value in the presoak, though I do it, but rather consider the primary benefit to be how it can provide constant flushing and lubrication at the point of wash medium-to-paint contact. *THAT* is what allows me to basically eliminate wash-induced marring and is why I use the foamgun.
I mix up gallons of concentrate and fill the foamgun with that. The mix I use doesn't really provide "thick foam" but rather the sort of soapy suds that work with my wash regimen. I mix:
~6-7 oz. shampoo with enough water to make a gallon of mix.
I use the two-three strongest settings on the foamgun. That combo of mix concentrate and foamgun setting doesn't appreciably affect my LSPs.
If using the foamgun the way I do, I'd sure replace the awkward pistol-grip with a shutoff valve (which I operate with my thumb). Eh, I'd probably replace it period, I just find it less comfortable than a shutoff.