You mentioned really liking how Qnap has a 3-tiered system (SATA drives, SATA SSD, NVMe SSD) but I haven't found an explanation of how it should be configured and the tradeoffs. Do you like Qtier? Should the SATA SSDs be a separate pool or merged with the main pool using Qtier?
Ok, this may not be a relevant answer but....
I previously worked in a company who made multi-tier storage systems with tiers like magnetic disk, optical WORM drives and tape tiers
tge architecture was structured on age of data with older stuff being migrated through to tape (offline).
that ageing may/may not be relevant for you but stuff that you need most should be in the quickest tier, stuff you need least in the slowest.
assuming it’s configurable to this extent, directory structure and thumbnails should therefore be in flash, metadata in SSD and images in SATA....I don’t have those storage options so I can’t check.
as I say, I don’t know if that’s relevant but to my mind, searching tags and seeing a thumbnail needs to be quick. This assumes you’re running a pic server, of course.
maybe you can adapt the above to whatever data you’re storing...?
of course, if you want the NAS or some loaded software to migrate content between tiers, then my answer above is no good. That sounds like a whole separate system to manage that.