A steel rod down every hole seems like overkill when a lot of these washouts show up right at the top 6 inches where you can just eyeball them. On the last three decks I dug, I ran the auger and then set a marked stake flush to grade, so any soft wall or cave in shows up as a gap you can see and feel without extra tools. The sprinkler line thing is real though, and a probe won't tell you it's a live line until water is already in the hole. I call 811 before every dig and still keep a shutoff wrench handy, that has saved me more than probing ever would.

Man, 9 spots over 14 months sounds less like bad luck and more like a you problem. Most people find a decent desk in a month or two, so maybe some of those dealbreakers were things you could've just lived with. Not every spot has to be perfect, sometimes good enough is good enough.