20 ohms through 50′ of cable is not within the acceptable range, 20 ohms at the solenoid is at the very low end of the acceptable range.
Lightning is something we get about once every other year here, and then usually at least 5 miles away, so no it is not lightning. At this location I seldom have power fluctuations. It is only the triacs and gate resistors that have fried. The only way for current to go through them is through the solenoid. The only way for enough current to go through the solenoid to fry it is if the solenoid is bad. I have six identical solenoids, and the resistance is significantly different between them.
It is the only plausible explanation that I have been able to come up with.
I have ordered a replacement Open Sprinkler Pi. I will put a fuse in series with the AC. I have also ordered replacement solenoids. We will see if the new unit survives more than a month.
I mostly do digital electronics, and some low voltage analog, so I am not too familiar with power electronics, but I’ll take a look, and see what solutions are available.