So it went down to -19C last night - I had the car plugged in, and while it was a hard start, it started on the first button push this morning. I drove to work, about a 5 min drive, then the car sat parked for two hours. At 8:30, when I went to do the school run, it took three tries to even get it to crank.
Getting back to work, I threw my NOCO Genius charger on it, and it went up to 100% almost instantly. I think this battery is just done. I've ordered a replacement - should be in this afternoon. It's just going to get colder, so I don't want to mess around with this anymore.