Interesting question! In regards to the Zone is that if you read ten different descriptions you will get at least five different answers... Not helpful but a good general rule is to go to a National Growers site like Monrovia for a good representation of the "average". If you are within a zone of them up or down 1 or 1 1/2 you are OK here.
Now if it has Wintered twice for you it is safe to say the plant is acclimated and within a safe range. Keep doing what you are doing

They can and do freeze many times during the Winter here unprotected but I don't allow the pots to be frozen solid for more then a few days. If it looks like they will remain frozen for longer period they go into the shed is all, nothing special, they just require window light or some light for the daylight hours.
Normally it is not needed and until this year have never had an extended freeze. For the first week they went into the shed to defrost a little, then to the attic, then back outside when the worst was over.
@GrimLore you have had success keeping some indoors during winter...correct?
I have grown certain types of Azalea for years in the house but they are considered "Greenhouse Azalea" in general, the types you sometimes see potted and blooming during the Christmas season. Those actually hate being outdoors even in the Summer and grow in controlled conditions only

A long time ago we discussed being able to force a few blooms a year with some light and water depravation and those were specifically what I was talking about then. Many other plants like Bougie in or outdoor can be controlled to with slightly different methods.
Grimmy
Edit: I must add that any of several types of Satsuki need some "chill" time for health and longevity much like Serissa and other plants...