I don't understand. If your PVB backflow preventer is freezing and cracking, you are winterizing it wrong (or it's been installed incorrectly)
I did this professionally for years, I've winterized hundreds of backflows, done correctly they won't freeze.
The only reason that commercial backflows use unions is so they don't' get stolen in the winter.

The install should have a shut off valve inside the house (or a stop and waste in the ground, less common but possible). There should be some type of drain between the shutoff and the upstream side of the backflow. open that drain. Leave it open, all winter. If the shut off seeps (common with older gate valves). the water will go down the drain, not into the backflow. Sometimes there's a weep drain on the actual shut off valve instead of a separate drain, leave that open. Open the petcocks on the backflow, leave them open all winter.

LEAVE THE BALL VALVES ON THE BACKFLOW IN THE OPEN POSITION. (biggest mistake most people make) and somewhere, on the downstream side, there should be a drain (or just a low zone works too), open that, let the water drain all out, close it up

water all gone, no possible way for water to enter it.

cannot freeze.

should take maybe 5 minutes to drain your backflow.

there is ZERO reason for unions or another failure joint.

If it's installed incorrectly (missing drains), then fix the underlying problem.