Yes, on the particular issue of company/corfu, I'm actually tempted to see if we can't just specify that the work of `completion-at-point-functions` must be done in an external process. Then calling `completion-at-point` would never directly call third-party code that may block the main thread.

Maybe for speed, `completion-at-point` would keep a fairly recent `dump-emacs-portable` image, and spin up external processes using that, with has the bonus that the completion-at-point-functions get to inspect current state.