Assigning different payment methods to Apple subscriptions

Correct. Only the payment methods on your on your account is charged. Generally you should not mix business and personal and it is recommended to have a separate iCloud account for business.

You can get something similar with multiple Apple Accounts and Family Sharing and individual payment methods.

Usual method for business and personal account separation is keeping those in separate Apple Accounts, as was mentioned.

Apple doesn’t have a feedback path for Apple Accounts and payment methods, and the closest is probably the iTunes Store:

Feedback - iTunes Store - Apple

You only alternative to the previous suggestions would be to carefully time the payment method used when a subscription is up for renewal. You can have multiple payment methods on an account and you would have to move your business one to the top of the list (and not have any account balance) so it was used when a business item needed payment. Then you'd move your personal payment to the top when something personal is about to renew. Yes, a hassle.

Here are articles about Family Sharing should you wish to investigate that. However, if you primarily use an iPhone you can only have one account at a time so it works best with two iPhones if using them.

Manage Family Sharing - Apple Support

How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support

Thank you. I get that, but as a solo self-employed person, it just doesn't make sense to have two separate iCloud accounts (or 2 phones to use them on).