How do i make sure that a subscriptions funds are removed from the itumes rather than the other payment methods?

You can try by making sure you always have adequate balance in your Apple Account. However, you may not have ultimate say as stated in How apps, content, and subscriptions from Apple are billed - Apple Support

Some purchases aren’t charged to your Apple Account balance

- Some subscriptions might not be charged to your Apple Account balance.

Apple provides no additional details about that.