Having reread the OP’s question (I now realise the full $4,000 has been paid one way or another), I think the $500 cash deposit paid on the second voyage is the one that will be forfeited with the potential of the $625 FCC also being forfeited. No doubt, this is why the question was posed.
As I see it, the 125% uplift on the first voyage deposit was compensation for Cunard not having to refund the cash and to promote future business. The fact that the OP wishes to cancel the second voyage where the FCC was utilised does not, in my opinion, reverse the OP’s agreement to accept the terms and conditions upon which Cunard offered the FCC in the first place. That said, it ought to be open to the OP to reallocate the $625 FCC (stemming from the first voyage that Cunard cancelled) to a third voyage providing this is done in accordance with the terms and conditions of the FCC which may, or course, include a term precluding such a transfer.
Putting taxes, fees and port expenses aside, any cash refund is only likely to amount to $2,875.