While I'm not in your area, we are in somewhat similar longitudinal locations for weather. By design, most SWGs will begin to stop producing chlorine when they hit 60 degrees or slightly lower. My water temp is just under 70 right now but often falls below 60 once the cold remains steady. Usually Nov - Feb/Mar for me. Once your SWG signals an error for low temp or simply stops producing FC, regular bleach is the routine. Even then, it shouldn't take much in the cold water as FC consumption drops dramatically.
More than anything, I'd remain vigilant on that pH with the new plaster over the winter. Test often enough to keep it in the recommended mid-7s range as that new quartz cures.