Samsung fridge-freezer RL37LGMH defrost problem

Hello, my 7 year old Samsung RL37LGMH fridge freezer (fridge over freezer) is causing trouble, and I’m looking for advice.

The freezer keeps clogging up with ice behind the back panel. (Works fine once I manually defrost it.) I presume the auto defrost is the problem, but how do I repair?

I’m getting normal ohm readings from the air temp thermistor and fan within the removable back panel. And if I look behind the panel, the heating element seems sound, with correct resistance also (235 ohms).

However I’m getting open circuits for both the thermal fuse, AND the other thermistor, which are both located within the iced-up coils. I’m assuming this is the problem, and to repair it I should buy replacements for these two parts but I’m an amateur so wanted to check this makes sense – I’m hoping someone might be able to advise?
​​​Thanks!