Selfhosted

If there's an offline backup, they could create a degraded RAIDz1 with the 2 12T disks, copy the data from the 6Ts over, create the 12T linear volume out of the 6Ts, add it to the degraded RAIDz1 and wait for it to resilver. If no hardware fails and they don't punch in a wrong keystroke, it should work.

Most USB enclosures and adapters aren't designed for 24/7 connectivity

This is true. I'm using 8 external USB drives in two RAIDz1s and I had to ensure their controllers don't overheat. For example I've had 4 WD Elements standing vertically, stacked next to each other. The inner two's controllers would overheat during initial data transfer and disconnect. Spacing then apart resolved this for my ambient environment. In the other pool, I had a new WD Elements overheat on its own, without taking ambient heat. I resolved that by adhering a small heatsink to the SATA-USB controller in the enclosure. I also drilled a hole in the enclosure immediately above the heatsink for better ventilation. I later applied this mod to the of the drives of the same model.

Mixing USB drives with any ZFS pool is a recipe for headache IMO.

Crucially however between the issues above and accidental cable unplugging, ZFS hasn't lost any data or caused any undue headache. If anything, getting back to a working state has been easier in some occasions as it would automatically detect a missing drive is back, resilver if needed and go its merry way. The headache I've observed most of the time has been of the sort - a message that zpool is not healthy, a drive has shown errors and/or missing, resolve drive issues if any, reconnect drive, no affected applications, no downtime. The much less often observed issue, probably twice over the last 5 years has been of the sort - applications are down, zpool isn't reading/writing or is missing, more than one drives is disconnected due to a cable snafu, shutdown, reconnect drives, boot, ZFS detects the drives and it proceeds as if nothing happened. All in, the occasions on which I had to manipulate ZFS over that last 5 years is around 5, most during the initial data transfer load. The previous LVM + mdraid setup I had required more work to get back in shape after a drive was kicked out for one reason or another. So yes USB can definitely present issues that you wouldn't see in an internal application, especially if some of your USB enclosure controllers are shit, but in my anecdotal experience, ZFS is very capable in handling those gracefully and with less manual intervention than the standard Linux solutions. If anything, ZFS has been less sensitive to hardware problems.