Frost everywhere except the heater - would you swap parts or diagnose deeper?

When I worked on a similar issue with a top-mount French door unit a couple of years ago, I defintely went for the meter check before buying anything. I'm a fan of not replacing components based on appearance alone. Frost buildup is a symptom, not a diagnosis of the failure. My unit's control board started acting up and the defrost timer was skipped, which led to the same excessive icing. It turned out the control board wasn't sending the trigger voltage to the heater at all, so replacing the coil woud have been a waste of time and money. I pulled up the service manual and tested for power delivery between the main board connector and the heater terminals. The board was dead on that output signal. It's always better to see if the board is telling the heater to run before you assume the heater failed. If the multimeter shows power is reaching the heater but it's still not melting the ice, then it is time to swap the coil. But if there's zero signal coming out of the control board, the board needs replacement. Testing the voltage drop across the coil is the fastest way to verify if the element itself has gone bad, assuming you know what voltage you should be seeing. Good luck with the repair.

I think you are focusing too much on the heater and the wiring, which are often symptoms. When I had a similar issue with a Whirlpool bottom freezer model last winter, the coils looked fine, but the interior air was just dead. I eventually realized the evaporator fan, which pushes the chilled air into the main compartment, had seized or been starved of power due to heavy ice buildup in its housing. It is not uncommon for the airflow to stop long before the heating element or the main circuit board gives out. This creates the perfect conditions for massive, unrecoverable ice sheets, making the fridge appear to have failed when it was just unable to circulate cold air.

On Samsungs there's a little defrost sensor clipped to the evaporator itself, and it's a known weak point. If that sensor goes out of range, the board thinks the coil is warm and never fires the heater at all, wich would match your evenly frosted coil exactly. I fixed my sister-in-law's Samsung two springs ago and that sensor was the culprit, the heater was fine. They're usually only a few bucks and pop off with a clip, so it might be worth testing or swapping it while you're already in there. Cheaper than a board if it turns out to be the problem.

I'd push back on the idea that a zero signal at the board connector automatically means the board is toast. That output only fires when the sensor clipped to the coil tells the board it's cold enought to need defrost, so a sensor stuck out of range will make a perfectly good board look dead. You'd swap a pricey board and the frost just comes right back. Easier to check the sensor with a meter at room temp, or jumper it and see if the heater actualy kicks on. That tells you which end of the chain is lying before any cash leaves your wallet.

Not much to add on the testing side, just wanted to say don't forget the simple stuff. My own fridge (a GE, totally diffrent animal I know) iced up the same way and it turned out the door gasket had a gap in one corner, so humid kitchen air kept sneaking in and the defrost could never keep up. Check the gasket with a dollar bill while you've got everything apart, and maybe check if the freezer door closes flush on its own. Might save you some parts swapping if that's the real culprit. This forum has been helpful just reading along, hope you get it sorted.