Sorry, they don't offer a discount instead of the gift card.
You can pay monthly installments early. See the following from How to make Apple Card payments - Apple Support
Make additional payments on your Apple Card Monthly Installments
If you pay your minimum payment due each month, you're automatically paying your Apple Card Monthly Installment for that month. To make an additional payment or pay off your installment balance, you need to pay your Maximum Payment for all other Apple Card purchases before additional payments can be applied to your installment balance.
Learn how to pay extra towards your Apple Card Monthly Installments.
Apple Card Monthly Installments are interest-free and all other purchases you make with your Apple Card have a variable APR.2 When you pay toward your Apple Card balance first, you can reduce or eliminate interest charges.
If you pay more towards your installment balance, you might reduce the number of payments, but you're still required to pay your installment the next month.