Okay, that makes sense. Thank you for sharing and helping!
In case anyone finds this later, yes, the 100 nF capacitor between the ADC pin and ground did instantly eliminate my noise issues. Oddly, noise was only present near 3.6V. Above and below (3.4V or 3.8V), it was fine. I expected more noise as voltage decreased, but that wasn’t the case. With the capacitor, very little noise across the full range. I’m still using 2x 200 kohm resistors.
Thanks everyone!