last decline reason is a good one, "insufficient funds" and "expired card" need totally different replies, one is wait and retry and the other is send them to update the card. do you store the raw decline code from stripe or map it to a few plain reasons first?

Yes, keep the raw decline_code next to a simpler category. The category picks the reply (wait and retry vs ask for a new card), and the raw code is what you need later when a pattern looks off, like a run of do_not_honor from one bank. If you only keep the category, you can't re-sort old failures when your rules change.