I agree with using a TA to get an onboard credit. I use
Dreams Unlimited, but there are many different agencies out there and some offer better incentives than others.
Yes, you can use a Disney gift card. There are options for getting additional discounts or perks when purchasing Disney gift cards. Target offers discounts for using your Red Card. There's a thread on this forum on how to get up to 12% off through Target. I can't remember all the steps, but it's worth looking up. I purchase my gift cards at grocery stores that offer discounts on gas for purchases. I pay attention to when they are having specials like double perks on gift cards and buy a bunch then.
Look into the Disney Vacation Accounts. They give you a $20 gift card for every $1000 spent from your account. You can fund the account with gift cards so you can combine the perks of buying gift cards with the extra $20 bonus from Disney.
As for which credit card to use, it really depends on what you want in return and if any of your cards are offering bonuses. The 0% interest for 6 months from the Disney Visa is a nice option if you are putting your cruise on your credit card rather than using gift cards. If you pay your entire cruise fare with the Disney Visa, you also get $50 onboard credit to use towards beverages. Then you have the 1% back in Disney Rewards that you could use towards either towards your cruise fare, excursions, or onboard charges like gratuities or alcohol. If you'd rather have the airline points, then use your SW card. I haven't really worked out if one is a better deal over the other.
Have any of your cards offered you bonus deals lately? Around last Christmas, I received an offer from my SW Visa for 5,000 extra points if I charged a certain amount within a 3 month period. I shifted all my spending onto my SW card and got the extra points. Many years ago, I used to get 5X rewards points offers from the Disney Visa card, but haven't seen one of those offers in forever. Earlier this year, my Chase Freedom card was offering 4% bonus on grocery purchases up to a total of $1500. So I went to Giant Eagle during one of their 20 cent fuel perks promotions and bought $1500 in Disney gift cards using my Freedom card.
There's lots of options here and you don't necessarily have to pick just one. For my last cruise, I paid about half with gift cards and used the Disney Visa for the 0% interest on the other half.