Is that all the frost you are getting on the evaporator coil?
If so, and your condenser fan motor and compressor are both running, then you have a sealed system problem.
The sealed system consists of the compressor, condenser, evaporator, heat exchanger, filter/drier and refrigerant. Common problems with a sealed system are bad compressors, refrigerant leaks, and system restrictions. Sealed system repair costs will often exceed the machines value.
I have basically the same problem. Temp is around 25 in freezer. Ice only on the top two coils of evaperator and thermistor is ice covered. Any chance of repair or should I replace? Is it possible thermistor is failed closed and heater is on constantly? Thermistor is in wiring harness and would need to be cut to be isolated. Removing two plugs and leaving other wiring in place shows continuity across circuit but can’t tell if it’s thermistor or other part of the circuit.
Thanks. That was my thought, but wanted to ask before buying a new unit. Replaced the evaporator fan a couple of years ago when the coil looked like your middle pic. Currently looks like the right hand pic... and getting warmer.