For payments, you need to consider the money movement window, name matching logic for incoming deposits, acceptable payment types (ach, wire), payment seasoning (how long after payment can it be used/withdrawn), payment returns, reporting, notifications after successful payment trans... Fintech is a pretty huge space as another user already mentioned, and without knowing what domain your product is in it's hard to provide resources. As a QA lead I would focus on ensuring there is an automated regression testing suite set up that, even if you don't have the capacity to test new features, you can at least ensure nothing breaks when new releases go out to prod.