Update:
I completed the transfer from my previous disk 3 over to the drive I intended to be the parity, then did a New Config. I confirmed that unRAID will accept my newly assigned parity drive. But unRAID also decided that the new disk 3, despite having been formatted XFS prior to the file transfer, was "unrecognizable" and forced me to do a format. I have once again disabled my parity and began another transfer of my last disk5 from my old array onto the new disk 3, after which I will re-add parity and cache to the array and let it do its parity sync.
Apparently one tip that I was given on Reddit is that, the next time I'm building an array, I should shave off about a GB or so off of the partition size for each disk, so that if I run into a slight mismatch like this, I have some breathing room. By my estimate, I believe the difference in disk size was about 250 MB or so.