There's also another approach: Not all PRs should introduce new, functioning features or complete rewrites. Thus you can just introduce a handful of functions, some of the functionality behind a feature flag, a new db schema... and then introduce the rest in follow-up PRs. GitHub even does stacked PRs for this reason, so you can start big, organize your commits into these chunks and do the stacked PRs.

Having AI generate massive code and shoving it to your teammates' plates should be considered irresponsible. Yes, test code is verbose, but it should be accompanied with an adequate description of what is currently tested. Otherwise it's just AI having fun in your codebase.

Automated PR merges could still work, please just don't with multiple thousands of LoC changes. Both LLMs and humans have a context size limit.