Sounds like your guess is right and its thermostat. Unless you have fitted the lower temperature one? When my thermostat was stuck open mine would not go above 70 on a cold night and would takes ages to warm up.
MMMM looks good , 340bhp at 0.6 bar would be nice... Did you find the front strut brace made much difference to how the car feels? Also how do you rate your H&R coilovers are they good for track use?
When i installed my standalone I just used the standard loom and cut out what I didnt need. Id imagine making a new loom for scratch wouldnt be technically hard but would take a lot of time planning to make sure you have all the right lengths and gauges of wire.
Definitely would be good to have a list of prices for basic services. Could also be worth practicing welding as always a few exhaust jobs im sure you could do for people.