When it comes to 10k LOC codebases, I still don't really trust it with anything. My best luck has been small personal projects where I can sort of trust it to make larger scale changes, but larger scale at a small level in the first place.
I've found it best for generating tests, autocompletion, especially if you give context via function names and parameter names I find it can oftentimes complete a whole function I was about to write using the interfaces available to it in files I've visited recently.
But besides that I don't really use it for much outside of starting from scratch on a new feature or getting helping me with getting a plan together before starting working on something I may be unfamiliar with.
We have access to all models available through copilot including o3 and o1, and access to chatgpt enterprise, and I do find using it via the chat interface nice just for architecting and planning. But I usually do the actual coding with help from autocompletion since it honestly takes longer to try to wrangle it into doing the correct thing than doing it myself with a little bit of its help.