This is of great interest to me as I rebuild my single group CMA lever.
Chert wrote: I placed another probe against the middle of the bell and covered it with metal ductwork tape. (TC2) BTW, that stuff doesn't stick too well
Kapton™ tape is what the Dr. Prescribes for situations like this. It has a strong adhesive, but comes up clean, is electrically insulating, and can withstand high (and low) temperatures. It's moderately expensive (per roll) but if you know an electronics lab technician, he may be able to give you a few inches of tape.
Regarding temperature control, I was contemplating adding a PID to the boiler. My thinking is that it will give better temperature stability and easier tweakability, versus the pressurestat. - the temperature would still put the boiler in the 1.2 bar range. I'd put the Solid State Relay in series with the pstat (set high) to keep the pstat as a safety cutoff. The Astoria boiler has that convenient drain plug - looks like an ideal spot for the thermocouple. But that doesn't give group temperatures.
But even with a PID, one needs to choose the target temperature.
I'm used to the HX flush on my other machine and adjust temperatures by feel.
This one is going to be interesting to figure out. At least I'm working in a controlled environment (indoors) and not lugging my machine around in all kinds of weather....