Given that it eventually works, I think you could be on target with a slow to react thermistor. Thermistors do have time constants, though I have no way of knowing the proper value for those in your fridge. Especially telling is the fact that you have tested the thermistors outside the fridge and found them to react slowly. Might be worth replacing one or both. I tend to suspect the Fresh Food Temp thermistor if you go that route.
Another thought, your fridge uses a TDM (Time Divided Multiple Evaporator) refrigerant cycle as far as I can tell. This means that a 3-way valve supplies the fresh food evaporator with liquid refrigerant when called for, and this refrigerant partially evaporates and then flows into the freezer evaporator and fully evaporates. (Called position A on the valve). The other position of the valve supplies the freezer only (Position B). It uses Position D (all off) to minimize gurgling refrigerant sounds when the compressor is off. (It doesn't use position C,all on) I am wondering if your 3-way valve is slow to react to calls for cooling in the fridge. You might want to replace the coil on the 3-way valve since maybe it is slow or sticks so it doesn't go from the all-off position (Position D) to the Fresh Food Evaporator (Position A) or hangs at the freezer only setting (Position B).
The defrost is an adaptive defrost that monitors operating conditions and schedules defrosts accordingly based on compressor run time and door open time. I do not think there is a discrete defrost timer, it is on the main control board.
Your exact manufacture date would be handy as some significant changes were made in October 2014.
It sounds like you have tech literature, but here's a [link|https://www.manualslib.com/download/763924/Ge-Adora-Dfe29jg.html|link to the service manual|new_window=true]