b01000100

OK, so I found another forum topic that addresses this.  I have no idea how I missed it the first time searching.  Let's blame it on the celebration last night.
 
Here is the forum topic that helped me: BTRFS CACHE FULL AND READONLY
 
I disabled Docker and VMs, stopped the array, restarted the array, and then was able to run the mover.  That will clear up some space initially so that I can take care of everything else.  
 
The key component of the post linked above was this:
 
 
Sure enough, that worked.  Once again, JorgeB to the rescue.  Thank you, itimpi, for the callout of no Minimum Free Space value being set.  I'll get that set now.