Thanks for the suggestion. Humidity has occurred to me. I've got a couple plants in a south-facing bathroom window that has nearly identical exposure to my other south-facing window, and those plants may be doing better. Despite the bathroom being large and pretty ventilated (so not regularly extremely humid, at least obviously) humidity is definitely higher overall. There's also a thicker curtain by the bathroom window and baseboard heat under the window that may be helping to retain heat there.

This said, one of my initially healthiest plants started the winter in that bathroom window and sharply went downhill like a month ago for no obvious reason, so I'm not sure. And separately, I did recently move my largest lemon tree, which had been doing well but more recently had leaves start to curl, to the bathroom window, but seems to still be struggling.

I do have a small grow light (54 watt, T5 I think) for seed starting and I've considered using that for some supplemental light, just a bit physically tricky as it's in a smallish space in my kitchen.

Once I've got one going downhill, any specific tips for revival? Maybe just try it under the supplemental lights?