My understanding is that you are entitled to a refund of any payments made for cruises that
Disney Cruise canceled, but that additional funds that
DCL provided cannot be refunded, only used towards payments made to DCL (for fares, onboard activities, excursions, etc.).
So in your scenario, you paid $6200 and DCL provided another $800 in future cruise credit good toward another cruise. $5000 of that payment is being applied to the January 2022 cruise. If you use the remaining $2000 toward another cruise and then cancel that cruise after taking the first cruise, you will get a refund of the $1200 that you have actually paid. You would not get a refund of the $800 provided as future cruise credit, because you can't get a refund of money you didn't spend.
I am not sure how it would work if you cancel the 2nd cruise before taking the first cruise. You would get a refund of the $1200 that you paid, but I don't know whether the $800 FCC could be moved to the first cruise as onboard credit, or if once applied to a cruise it must stay with that cruise or a later cruise.
If my understanding of what is refundable is incorrect, I'm sure someone will correct me.