yeah that sameness is nasty — short on funds and expired card look identical to the customer too, so they don't know which fix to try. asking for a card update in the renewal reminder is a smart place for it, before anything has failed. do you put that ask in the same reminder email, or as a separate note a few days earlier?

Exactly — once the failed-payment email is one template for both, the reminder is the last clean chance to say "update the card" before they guess wrong.