GE Profile SbyS defrost control: temp problem

Seems a common problem. Freezer cold (goes to 9 degrees, never lower) and fridge temp shows 123 degrees (It really is very very hot). Happens immediately, within 1.5 hours of being plugged-in. Changed main board. No improvement. Most comments say problem is a defrost problem. Want to change heater, thermostat, and defrost control. All other fridges seem to have defrost timer. Can not find one for this model in parts diagram. Where is defrost timer/control? Can the part be purchased, part number, if yes? Fridge has electronic touch controls for temp, etc.

Am I barking up the wrong tree, i.e., is there another/other reason for problem or suggestions to fix the problem? Seems weird that it is a defrost/heater problem. Seems like there is insufficient time for the evaporator coils to frost-up. Stumped. Hate to toss it. It’s been really good up to now and retirement budget is tight

Thanks for any and all help Bill