Note: the worn drives were very worn. 128GB drive with 280TB written is ~2400 cycles. >5x it's 480x rating!

Even though it's a cheap drive, it's rated endurance wasn't really that low. 600 cycles (1200TBW/2TB) is pretty common for consumer SSD: that's higher than 480x but not vastly higher.

Glad folks are chiming in with the temperature sensitivity notes. I have parts coming in for a much bigger home-cloud system, and was planning on putting it in the attic. But it's often >110°F in the summer up there! I don't know how much of a difference that would make, given that the system will be on 24/7; some folks seem to say having it powered on should be enough, others note that usually it's during read that cells are refreshed.

Doing an annual dd if=/nvme0n1 of=/dev/zero bs=$((1024*1024)) hadn't been the plan, but maybe it needs to be!

Just my 2 cents: I've installed quite a few servers and racks of hardware in very unsuitable spaces. LTO drives in dusty rooms, important disks in hot and cold (summer and winter), dusty and vibrating rooms and so on. I was very worried about them dying way sooner than people would expect. The only thing that died after two years was the LTO drives. Everything else is still running. So I wouldn't worry much about your hot attic. 43° Celsius outside means constantly over 50° inside the server, which sounds horrible but in the last 15 years of my experience nothing died because the room was too hot, or cold, or humid, or shaky. I've installed dust Filters in front of all air intakes, though. My reasoning is, that dust kills way faster than temperature.

Ok I gotta ask...why of=/dev/zero and not of=/dev/null ?

15 years ago the average MLC flash was rated for closer to 5k cycles, not 600.

The fact that even the "fresh" drive, with just 1 cycle, is already showing some degradation is also concerning.