Thanks, LaPastenague. We're getting a little above my pay grade here.
Here's a fuller account of the situation. The 2 TB Time Capsule backed up 4 machines via wifi Time Machine:
MacAir mid 2011 with i5 and 2GB RAM, now running 10.9.5 with 64 GB SSD storage
slightly newer MBP whose battery ballooned about two years ago
iMac mid 2010 with i3 and 4 GB RAM, now running 10.8.5 with 500 GB old style storage
MacMini late 2012 with quad core i7 and 4GB of RAM running 10.13 with 250 GB SSD storage
I did email, work, and other daily activities primarily on the MBP until its battery died. At that point, I wiped the MacMini and used wired Migration Assistant to move all photos, etc. from the Time Capsule backup of MBP to the MacMini. No known loss of data. Then backup MacMini. Time Capsule use (total for all machines) was still well below 50% of capacity.
Now scroll forward to Dec 2019, MacMini having been the primary activity machine for about 18 months, including an update or two. A client nagged me about providing an invoice in .xlsm format so I downloaded MS Office for Mac onto the MacMini. A few quirks began to occur, such as problems with outgoing via Mail, and didn't go away when I deleted after the 30-day free trial due to M$oft new policy of renting software instead of selling it.
But still no problem with manual backup to Time Capsule, occasional review of the disk images, rare restore of a file or two.
Some other quirks in Jan-Feb, with persistent spyware intrusions of a program whose icon was a magnifying glass on a blue-green background. Easy enough to find and delete. A friend who used to write Mac articles for a major daily newspaper advised wiping the MacMini and clean install of a new OS. Internet install pushed me to Catalina.
No apparent problems saving to Time Capsule via the home wifi use of Time Machine for all 3 devices -- could see disk images when scrolling thru dates. Total usage of the storage for all machines combined was about 750 GB.
Then the install of 10.15.3 and blank directories, grayed out disk images. A couple days later, no dates visible to scroll at right of screen when Enter Time Machine -- for any of the devices. IDK how but Catalina seemed to modify the entire Time Capsule directory of bundles, blocking (or possibly corrupting) hundreds of backups of the older non-Catalina devices, too.
I don't have an external hard drive, so I'll have to get one and archive rather than hold my breath for an update that it seems a lot of other people are also hoping for (and consumer demand is often a factor in prioritizing updates, no?).
At least Mail works fine on the MacMini now...