Secondly, how often (if you know) does a backflow that’s tested show a problem?
I’ve not heard of one failing, but I’m sure that doesn’t mean that they don’t ever fail. Your water system manager or operator might be able to tell you.
You might also check with them about how to disconnect your yard sprinkler system to the degree needed to ensure them that it isn’t being used - that might be the avenue to getting away from a yearly backflow prevention assembly test...