How about manually adding credit to your Apple account for a smaller amount that the annual subscriptions, to use for in app purchases etc. Like $50 in this example.
Then manually top up the credit to $50 when it gets low. Can use any payment method to top up or use Apple gift card.
Maybe even automatically top up from a payment method that isn't linked to your Apple account (have to check if that's possible.)
Then you only need to have your payment method for annual subscriptions linked to your Apple account.
The annual subscriptions should be more than the $50, so it will revert to using that for the subscription amount.
Yes, it may use the credit first and then the rest from your subscription payment method, but you could just pay that smaller amount back once a year.
Just an idea that I'll be trying myself.