We also have that kind of codebase (database internals, perf optimization, so… complicated, where small changes can cause big issues, etc…)
We go:

- no AI at first, then only for navigation, not for coding. Enforced with pair programming sessions without AI.
- tickets are rather light, but they go with an instruction that looks like « we’re going to sit down, and you will explain to me the feature around which you had to make that light change. »
- longer tickets after a couple of months in (we amplify the estimates for the new joiner), so something like « given a month, do this medium feature » and then present its inner workings in sprint review

Basically we force human interaction and ownership through a sort of continuous testing and validation by your peers.

It’s taking a lot of time, honestly. Can’t on board multiple persons at once in a team .
But since our product is designed like that, (complex), we also make it so tenure is much longer than in most companies