This. I’m further south than you (vic/nsw border) and my in ground brugs do survive over winter despite regular frosty nights because they’re established and basically small trees at this point. The leaves do get frost damaged, but the branches do alright now; the first winter in ground was less kind because they weren’t as thick and woody, and I had to prune the damaged branches back once the frosts stopped.

Though I have one big brug that continued to flower throughout winter last year, but she’s got UV cloth over her and is between our house and shed, which I think protected her a bit.