Changing my primary card for subscription

You can reorder payment methods so one is the primary method, but account balance will always be used first if possible.

From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

How apps, content, and subscriptions from Apple are billed - Apple Support

Some purchases aren’t charged to your Apple ID balance

● If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method.

● You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

● Some subscriptions might not be charged to your Apple ID balance.

As for the payment order issue, I don't have any payments on my account so I can't check myself. I've seen many posts saying that's how you do it. Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support