"lead the water out to the sprinkler with freeze proof plastic tubing, and switch to either copper or galvanized for the last 10 feet or so."
I can't see any reason for going from copper to plastic back to copper. The plastic buys you absolutely nothing and the unions are generally a pain in the ass to work with compared to a straight run of copper.
Or, make one union and go all plastic.
The "copper to galvanized to stainless, to bronze, to plastic, to carbon nanotubes, to PVC to CPVC to ABS to copper" doesn't make a lot of sense.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.