Thanks for all the replies. First, if I had known about it before I began, might have used the OCLP method. Basically chose Catalina 15.7 because in order to do my taxes, HRBlock this year has that as minimum sys requirement. So I already have a working Catalina Patcher after using DosDude instructions.
https://www.macrumors.com/how-to/install-macos-catalina-unsupported-mac/
Main reason for wanting to have the option of a bootable backup is that this is an old Mac with the original spinner drive -- many hours -- so in case it dies (planning to replace with an SSD at some point) I would still be able to continue on the backup on an external drive. Also, if needed, want to be able to copy any file changes back to both System and Data on the Patcher volume -- which, unlike on the High Sierra Desktop, shows both volumes merged.
From HS Desktop:
View attachment 2142993
Note, the 2 Macintosh HD volumes, system and data, were generated after erasing and creating a new partition (APFS) on a Mojave Patcher volume with room to spare, then running the DosDude instructions with the usual Flash drive procedure. And the two external volumes created from running CCC from the Patcher to another erased now APFS formatted external volume.
Re. CCC, CCC Lets me copy the external data volume to the HD Catalina Volume, but not the system to system. Gives me the following message when I try
View attachment 2142994
Also get the no entry sign when I try to boot the Catalina from the external, either from Restart>Startup Disk or from Restart>option (boot picker). At that point have to completely shutdown and restart.
So back to square 1, does this mean that, in order to have a bootable backup, I do need to create another empty Patcher volume (somewhere) and then use CCC to clone the original Patcher to that? Or, taking the preceding caveats into account, is there some other way?