Your trouble tree indicated it could be the defroster timer, the defrost coils or the thermostat. I chose to replace the defrost timer first since it was the easiest to replace. Your video showed how to replace the timer and it went smoothly.
While waiting for the timer to arrive we unloaded the refrigerator into ice chests and the freezer side into another freezer we had. We turned the power off to the refrigerator and let it defrost. The evaporator coils were encased in ice. Once it was thawed out we restored power and let the refrigerator cool down before refilling it. It ran well until the timer arrived since it takes several days for the evaporator coils to get coated in ice to the point the refrigerator performance is degraded.
This is the third appliance we have repaired with your help. We also replaced the control board on the electric oven and the thermostat on the electric clothes dryer.