@Tomcooper If you want to truly winterize your backflow preventer, you will have to take it apart. The device will hold water in the check valve chambers. Then, you should have the backflow device retested to make sure it’s working properly after rebuilding.
I like your idea of draining though. The other thing to think about is refilling the mainline when it’s empty. If you turn on the master valve to fill the system at full pressure on an empty mainline, you could have all kinds of water hammer damage to joints.