We're in a similarish situation. We decided to accept spending a bit of time on the documentation every time some JIRA ticket requires us to work on an undocume

We're in a similarish situation. We decided to accept spending a bit of time on the documentation every time some JIRA ticket requires us to work on an undocumented section of code. Not everything requires a lot of documentation, we have a lot of essentially glorified input dialogs, but we do try to write down what it is, and especially any logic and config settings that affect that logic. One thing I've used before with success and which we've also introduced was two new fields in JIRA, "requires documentation update" and "documentation updated", to aid not forgetting to update documentation when adding or changing code.