Thanks, John, for your feedback. You are completely correct about Boot Camp Assistant. (In addition, Boot Camp Assistant also requires the Windows Installation ISO image, which I did not have handy.) I did not find any means to expand the Bootcamp drive in situ. I also tried Stellar Partition Manager, but it did not even recognize the existing Bootcamp drive, and I'm still waiting for a refund. I did see elsewhere online that others found that Paragon's CampTune did not work for them, so I did not attempt to use it.
I ultimately used Winclone to back up the Bootcamp drive to my external RAID, then I deleted the Bootcamp Drive from within the APFS container on my (recently backed up) startup drive. I made a new ExFat drive at the new, larger size, then used WinClone to copy my block-based backup onto the new, larger NTSF formatted drive. A re-boot of each, and the new disk appears as a Bootcamp drive in the Finder and Disk Utilities and everything works fine. It was just a little unnerving to delete the existing Bootcamp drive without complete confidence that I could re-build it. I did test the backup Winclone drive first.
It seems the game has changed since Apple switched to the APFS volume structure with MacOS 10.13. There is a comment in Wikipedia on this, as follows:
https://en.wikipedia.org/wiki/Apple_File_System
Despite the ubiquity of APFS volumes in today's Macs and the format's 2016 introduction, third party repair utilities continue to have notable limitations in supporting APFS volumes, due to Apple's delayed release of complete documentation. According to Alsoft, the maker of the popular DiskWarrior, Apple's 2018 release of finalized APFS format documentation will enable safe rebuilding of APFS disks in future versions of its flagship product.[28] Competing products, including as MicroMat's TechTool and Prosoft's Drive Genius are expected to increase APFS support as well.
Again, I really appreciate your feedback. This was a vexing problem, and most of the information available online relates to expanding Bootcamp drives in situ prior to the introduction of APFS volumes.
--Rich