just a little note because some people just can't read (i don't mean you nnsqnap):
support volumes >16 tb
, absolutely no problem! everything's fine as marketed. you can create for example 8 x 3 tb to get 21tb usable space in a raid5. or 33 tb in the 12 bay nas.
the limitation is just the expansion of existing volumes
without loosing data, which has been noted multiple times before (and by qnap on the website). if somebody does a new setup, it's completely irrelevant as no expansion is needed. and anytime you can delete the existing <16tb volume and create a new >16tb volume - of course then you need to backup the data first. but if somebody uses a +/- 16 tb storage device without doing a regular backup anyway (if it's not the backup already), i would really question that person's expertise.
All that said, is there a way to expand beyond 16TB from command-line while preserving the existing data?
i'm not sure, but so far i didn't stumble upon that possibility here in the forum (although i may have missed it of course, let us know if you find something). afaik the linux/ext4 file system tools just can't do it yet (if that's still correct?), so not a qnap-only problem.
Also, has QNAP officially commented on the issue and when / if they might have full functionality to expand beyond 16TB via the existing GUI tools?
didn't see any statement on this so far. guess it's difficult because it doesn't depend upon qnap only.