Most critical for real money scenarios imo is to find the requirements for and designing a ledger. The most urgent quality improvements may be derived from that to an extent.
A place to look for ideas is “event sourcing”. It’s a pattern where your source of truth is not mutable tables (the typical DB design) but an immutable array of events. It’s particularly well suited for situations where you need auditability. And using it should be familiar if you have used for example redux for reducing events to an app state.