I think the only way is to start switching gradually. If you don't want Google to read your email, keeping it as the main accounts isn't really going to help.

If you do change, get a forwarding service or your own domain so that the same mistake doesn't happen again.

Isn't vendor lock in great? :) This is why I started using my own domain for emails a long time ago. Still have some stuff on my gmail, but I've mostly broken free.

I did the switch a few years ago. I setup forwarding after importing all my emails from gmail, then i spent months changing email on services whenever i used them. eventually you finish and can stop the forwarding or close the gmail account completely.

Try IMAPsync? http://imapsync.lamiral.info