I think it would be easier for you to purchase the
RAPT Temp controller 15amp model then:
1. Remove the Cooling output
2. Re-wire the heating elements to the RED plug on the RAPT temp controller
3. Then just use a manual switch to control the pump
4. Run wire from the temp probe on the guten to the temp controller. You probably want to re-calibrate after this is done. If the guten uses a strange ohm reading you may also need to replace the probe with
this one.
The other way is to upgrade the guten with the
BrewZilla Gen 4 controller and then upgrade the main
gen 4 powerboard. The wiring on this is a bit more complicated so I think the temp controller would be a bit easier. I am not sure how easily you will fit this board and gear into the Guten either so the temp controller might be safer as most of the electronics would be external to the body of the unit.