"The rub is that my High Sierra boot drive is one I want to clone to a bigger SSD. "
There is NO ISSUE here.
There is nothing "additional" or extra you have to do.
CCC will clone from a smaller drive to a larger one -- it doesn't care about size.
You can even clone from a 1tb drive to a 500gb drive -- IF you have used only, say, 300gb.
The drive SIZE is of no importance.
What IS important is HOW MUCH SPACE has been used up on the drive.
Got it?
I think you have gone out of your way to do all of the above "for nothing".
All you really needed to do was connect the smaller drive to the larger one and... clone away.
Thanks and I do understand. My point was that I have 2 SSDs and I can't immediately clone one to the other without losing the system I clone to, unless I make a copy first. So I made a copy volume for each on HD partitions, just in case something goes wrong. As it is, there is a problem with the High Sierra SSD as detected by Disk Utility and Drive Genius.
node_val: object(did 0X3):invalid children(-2)
I will make a new backup and try to fix. Probably I will need to reinstall (dreading) as some cursory repairs didn't solve it. Unfortunately I formatted my intended new and larger boot SSD as APFS and now can't go back. I think I'd have been better off with HFS, but at least it will be Mojave-ready.