Update: my plant is showing signs of bouncing back! He's lost almost all of his lower leaves, but his upper leaves are perking up and he's looking happier and happier. I watered thoroughly yesterday (9 days after the last time I watered) and I've kept the humidifier running. I will continue to monitor and make sure I stick to a consistent schedule of watering (and watering until the water drains).
Will-you're right. The sensor seems off! I will not follow it but just stick to consistency and feeling out the soil. I believe what saved him is the bright window he's next to (and the fact that it's been his spot for as long as we've lived in this home).
Thank you all!!