This is all pure speculation (I have zero experience with artificial light) but doesn't it make sense to have actual short days (<12h) during winter? Maybe that little bit of extra light is helpful? Or maybe it doesn't matter given you stop watering for a while. I know that short days are a seasonal trigger for some plants. I am surprised you have to dial back the watering so far during winter given the ample light you are providing. Your plants look amazing, so keep doing whatever it is, but I am surprised about the winter care.
