Nick, you hooked the mix tank bypass up wrong, it's impossible to get 5 gal that hot that quick with a constantly cycling system. and the thermal relief is not an option but part of the system, when it pops it dumps 30 oz. or so of hot and lets the box fills with 30 oz. or so of incoming cold, resetting the clock, then all it takes is a good pull of the trigger on the wand and you further reset the clock.
If you only ever set the wand down for a few mins. It would NEVER bypass to the tank.
I could let mine run for 15 mins. wide open without it cycling even, and it takes several "cycles" to build up enough heat to be a waste tank dumping issue, it's just not a problem.
If you do flood work, I would add a manual exhaust diverter, it's bullet proof.
The point is, don't post about something not working when I know for a fact it does. I suspect right away, you were also bypasing too much water to the box, did you use a needle valve to "dial" it in, or did you just use some crude fixture like a ball valve to control it, it only takes a small amount to keep everything cycling.