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.
No, this is trying to solve the wrong issues.
What's the internet connection like?
How do you currently store files? Qualify what you think is large - 10gb, 100gb, 1tb. What is a project size currently? How many cameras & at what resolution/codec?
What editing tools are you working in? Have you looked at cloud editors that allow for collaborative remote editing?
Do not install TrueNAS on the Mac - it's not going to work. A dedicated NAS like a Synology will be cheaper than anything done on the Mac. What is the total storage you have currently & how much does it grow per month?
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Splitting things between a read-only share (of things that have been archived to tape), and a normal working share would help on the workflow.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.