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.

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.