No, do not do anything with the small HDD’s. If you want a backup drive, get another 4tb SSD - they’re cheaper than anything you’d put the multiple drives into & will run without being plugged into power.
You do not want sync, you want backup. Sync means if you delete something, it’s deleted from the HDD’s as well. Or save over an existing file.
Mac or PC?
No, do not do anything with the small HDD’s. If you want a backup drive, get another 4tb SSD - they’re cheaper than anything you’d put the multiple drives into & will run without being plugged into power.
You do not want sync, you want backup. Sync means if you delete something, it’s deleted from the HDD’s as well. Or save over an existing file.
There isn’t.
3.5" HDD’s or 2.5" SSD’s are your options.
You have 3 issues, online archive of past projects, long term (offline) storage & client access.
LTO is your long term solution for offline archive of projects. Depending on the average / largest project you might want to do 1 project per tape so LTO7/8 sizes. Scales really well, multiple copies, etc.
For the online storage, a NAS is really the only option. How it’s sized & configured comes into play. You can go cheaper with used enterprise gear, but then you’re dealing with more disks & higher power bills. Fewer larger disks can help with the power bill & noise levels.
Splitting things between a read-only share (of things that have been archived to tape), and a normal working share would help on the workflow.
The catch is what you do for client data exchanges. Giving them access via Dropbox is nice, but you need better housekeeping around data. Once the 1 year grace is over, what’s the size they have committed to? While self-hosting a client accessible share is possible, there’s ongoing costs & I would be cautious around exposing the NAS to the internet directly.