davemark, 
Rephrasing my question:
If I turn off iCloud (say, I stop paying the monthly fee), I assume I will lose access to all files in the cloud but NOT downloaded to my Mac.
Is there a way to backup ALL my Mac's files, both on device and in the cloud?
Happy to buy a large enough drive. My concern is that Time Machine only backs up what's on my device, depends on iCloud to back up the cloud stuff.