My entire NAS is used parts, including used enterprise HDD’s. I put the gear through a thermal, memory, and IO stress tests and all checked out. I bought 10 drives when I only needed 8 with assumption that at least 2 would be bad (none of them were and they’ve been in use for > 2 years). Correction - the case was new, I uses a Rosswill rackmount.

Internally - there is basically zero difference between a usb thumbdrive and a microsd card.

Just a thought on electrical grounding issues - Any GRND imbalance between the external PSU and the server will result in a small current flowing through the card via the GRND traces. In a perfect world this should be negligible, however, to play it safe I would make sure the server and PSU are plugged into the same power strip at all times so they share the same ground bus bar. If you have a multimeter available I would also verify that the voltage difference between the grounds on the DC side of the 2 PSU’s is exactly 0 is very very close to 0v.

As far as backing up your LUKS keys. This may sound low-tech but print a QR code of the keys onto paper and put it with your other important physical documents.

When I got my dashcams, which is the hardest duty for SD card. Constant writing and massive temperature swings. I used 32gb cheap cards, they failed within a month. I replaced them with name brand cards, again, all 3 cameras failed within another 2 months. I’ve also had SD cards fails multiple times with my RPi. The only solution was to buy the High Endurance SANDisk cards. I’m 8 months in and they are still going. But I expect them to fail soon enough. SD cards shouldn’t be used for reliable storage, period.

Buy a few 16 or 20tb external drives on a credit card from a local store, use them, then return them. It’s morally grey renting.