Offsite Backup Strategy for Multiple Macs
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.