I like doing this math.

A DS1821+ with (2) DX517 expansion bays would cost 4.1K AUD presuming 10% tax and would be 307 TB presuming (18) 22TB drives with a BTFRS file system running SHR-2 (allows for 2 drive failures).

(18) 22TB drives @ $22/tb AUD = $9.5K

So an all in cost for 307TB is 13.6K AUD using that equipment. 27.2K AUD to have a mirrored backup, but it sounds like you're ready for another 300+ TB right now, so 54.4K AUD to have 1:1 backups and 307TB of runway.

If AWS Glacier is what you're comparing to, then you make that up in 6 months.

Rack mount would be more convenient, as you can have 1PB volumes and a little less cumbersome and tidy setup - the 1821+ with expansion bays are 108TB max per volume, so you'd have to deal with 6 different volumes but maybe not a big deal if your filing system is by year/month. But getting into rack mount with Synology for example would basically double your infrastructure cost. Or you bite the big bullet now on scaleability and use a 60 Bay rack mount @ 29.9K AUD for just 1, but it's still roughly the same cost per drive bay as the 16 bay.

Suggesting SSD/flash for archive “cold” storage is uh, not good. SSD/flash drives are temperature sensitive, and have a lifespan of no longer than 2 years if they are not turned on periodically. For someone looking into optical storage, I’d bet a healthy wager they are looking to put some of the most important data in a closet somewhere in case of their traditional HDD setup(s) failing catastrophically. It’s not a bad idea. HDDs are also fine for “cold” storage as well, but they’re still mechanical, and having something mechanical sit there for years and turned on after a long period of time sometimes doesn’t work out well.