Let me repeat what I'm saying for the last 3+ decades
At some point it's better to look at an old code base, make sure it's as thoroughly documented as possible (esp. the foot guns) and do a clean reimplementation without lagacy cruft.
DO NOT implement new functionality into the old codebase but implement it in the new one and backport from there when needed.
Regression testing and corner case testing shouldn't be done by the same team.
Edit: Under no cirmcumstances can an LLM be used for any of that.
