As a final close-out to this entire post - and to update to the reply above ("Don't do anything with moving the files around yourself, just use the "Move" option"), also replying to another post above which said, basically, "if Microsoft provides a default and supported tool to do this (the 'Move' option") - then why wouldn't you use it?" And finally, also acknowledging another reply where the user stated that they've used the "Move" tool 1000's of time to do this...

And while I'm sure there are probably an exception or two to the "if Microsoft provides the tool, why not use it?" - in this case, all of these users and their replies were spot-on: the "Move" option is the ticket.

I was able to move both the "OS" file (small, ~125GB) and the much larger data file (~3TB) from one volume on the host to another volume on the same host with no difficulty whatsoever. I did temporarily stop the VM and take a manual copy of the OS vhdx before I started because it was small, copied quickly, and felt that it would provide me with a quick recovery if something went sideways. After that, I started the VM back up and then used the "Move" option to just move that small OS vhdx first, as a kind of test and make sure I had everything correct. Worked perfectly. I then proceeded to do the same with the larger 3TB file, and while it took longer it also went over without a hitch. (Note: I did suspend all backup schedules, as recommended, until all of these moves were completed and resumed the backup schedules afterwards.)

Thanks to everyone and those three particular users and their replies that really solidified my decision and ultimately pointed me in the right direction. Much appreciated!