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Point taken, and I never said I WOULDN'T install a backflow preventer. But that wasn't my question. I don't have questions on whether or not a BFP is necessary, I asked for thoughts on improving my current design.You do not have that right. Unless your state has no construction codes whatsoever, there are plumbing rules to follow, and backflow prevention is a part of those rules. A dual check device in place near a water meter is meant to be insurance to protect the city water supply from dipsh!t homeowners, but it is not rated as protection from lawn sprinkler systems.
Consider the practical matter of someday wanting to sell your home. The structure will be inspected. Unsafe plumbing gets flagged, and WILL be corrected to code official satisfaction, with fees and maybe fines piling up in the process.
If your sprinkler system is so marginal in performance right now, and you're willing to expend time and money to improve it, spend the effort wisely and install a PVB (if you are on flat ground) or an RPZ, and make the system work with the altered supply performance. There is absolutely no good reason not to do it.
By the way, if you did pull out working Rainbird R-50 heads, you can probably sell them on ebay.