Offsite Backup Strategy for Multiple Macs I have 4 Macs in my household that have a decent local backup strategy.  I am preparing an offsite backup disk and wondering about the best strategy for structuring the offsite backup disk.

I want to protect:


4 Macs with a total of 3.75 TB of internal storage.  Approx. 80% usage of capacity
2 external drives connected to two of those Macs


5 TB capacity, 50% used
4 TB capacity, 25% used



Potential capacity to be backed up is 12.75 TB, but current data size is 6.5 TB.  I have purchased a single 8TB external drive to be taken offsite.  Yes 12.75 TB cannot fit in 8 TB, but 6.5 TB can.  I don't foresee the external drive usage to increase more than 1 TB in the next year.

Caveats

Backing up to the cloud is out of the question, as I am working with a 100 GB per month data budget.

Requirements


All of the data on the external drives is vital data and large, such as my photography.  That alone comprises 2.2 TB
Everyone in the household makes their living from their Macs.  If a big disaster happens, I need to purchase new Macs and get them back to fully functioning in the shortest time possible


Backup format

Each Mac is currently backed up locally by both Time Machine and a bootable backup via Carbon Copy Cloner (CCC) to separate disks.  Since I have these set up already, I'd prefer to stick with these options, but am open to suggestions.

CCC would require partitioning the backup drive into 6 partitions, locking in the amount of data I can backup up for each source.  Time Machine (TM) on a single 8 TB partition would allow the free space to "float" to where it is needed as the external drive usage increases. Because of this, I am leaning towards TM vs. CCC simply because of the amount of data to back up.

Backup Procedure

I would be storing this 8TB disk offsite and bringing it to my house occasionally.  I would then connect the drive to each computer, let TM do its job, then take the disk off-site again.  I may put a second disk in rotation so that there is never a time when all disks are on-site.  See my answer to a similar question https://apple.stackexchange.com/a/381886/146721 for the details of how the rotation works.

Encryption

Encryption is a must since I cannot guarantee the physical security of the offsite drive.  If I go with the TM route for backup, I can encrypt the entire drive with a single password or allow each machine's backup to be encrypted with a separate password.  Since these are family backups, I am not terribly concerned with intra-user security, so I am leaning towards a single encryption password.

Restoration Scenario

My existing in-house backup strategy (TM + CCC) should take care of most mundane restoration situations.  The scenario I am attempting to cover here is complete disaster, such as the house burning to the ground.  In this case, all computers and on-site backups are assumed to be destroyed.  In this scenario, new hardware would have to be purchased and restored from the TM backups

Does this seem like a reasonable strategy?
I think you're making this too complex.
The very first thing you need to do is decide what's more important - an image of your entire system or your data.  For all of the clients I've worked for, data was the most important factor.
Your settings and applications are important, don't get me wrong, but I'm sure you're much more concerned about your tax documents than you are about your custom shortcuts you've created.
Given that your Internet connection is metered, you can still use the cloud, you're just going to use it differently.  Here's what I suggest:

Time Machine
Cloud Service
NAS (with USB based backup)

Time Machine.
Use Time Machine for your whole system backup.  You can rotate disks if you like, but I've found this unnecessary over the last decade.
Cloud Service
I personally use OneDrive, but this will work well with DropBox or other similar services.  In my Documents folder, I put symlinks to various folders in my OneDrive folder (it's the one that gets synced).  I don't copy over Pictures (they're automatically uploaded from the phone) or Downloads, but you can if you want.
NAS or Network Attached Storage
I personally use a Synology NAS with a several USB drives attached to back up the NAS - one for music, another for documents, and one that backs up the whole NAS.  The beauty with a NAS is that all of your Macs can attach the NAS and benefit from it's features.
Now, here's where the magic happens...  The Synology NAS syncs with OneDrive.  The NAS backs up to at least two places (i.e. Documents and whole system).
So, what does this accomplish?  Let's look at that tax document...
Document 
  → Backed up via Time Machine
  → Sync to One Drive
    → Sync to NAS
    → Backed up to USB (Documents)
         → Backed up to USB (Whole NAS)

With a setup like this, you have your data in 6 different places including off site.  By not backing up your entire system to the cloud, you're minimizing the data transfer as it will only sync the changes not the entire data directory and certainly not the entire drive.
You said you had a 100GB data cap on your Internet connection.  Once this is setup, I highly doubt you will transfer 10GB of documents and data per month.  I work from home with three other people in the house and don't hit that level.
Here's a similar answer where I didn't use OneDrive in the equation.  I backup my VMs to my NAS, but I don't sync them to OneDrive.  What this allowed me to do when my Mac crashed was to bring up my dev servers on an inexpensive Dell running FreeBSD (it could have been Windows) and VirtualBox and get back to work immediately.
Finally, the NAS setup allows you to centralize all of your backups for Macs because it acts as a Time Machine server, you'll just need to size it appropriately.