The trouble is I have > 10 years of mail sitting in the OSX mail system and although I could probably export it all (it at least used to be relatively straightforward to do this) the sheer terror of doing that is, well, terrifying because there's a lot of stuff in there that matters.

Using the system-provided mail tool was a stupid decision, and one I managed to avoid with the browser &c, but it's too late now.

(Now this is an off-topic discussion from a discussion about off-topicness.)