Coating would work, but it needs to be permanent, because you can’t easily recoat it. Glass can be recoated on the other side. Dunno how anodising and etching would work, do you get the same color when you etch, or do you get raw aluminium?
I suppose if you scanned the plates in a flatbed scanner that might be even contrast, I was thinking of using a phone camera, and assumed it would be near impossible to get an even lighting across the QR code.






Rtx3050? Should be supported for a little while, and seems to be around $300AUD. Probably still overkill for a NAS though.