[pwg]

> if I have my own server, but use Gmail as client, would google still read my data?

The answer here would be "yes", at least from the "is it possible" viewpoint.

> If you have migrated away

Never had to migrate away, because never utilized one of the "free providers". When I purchased my own domain (circa 2001) I self hosted my email from the start, and it is still self hosted.

Do note that if you communicate with anyone who uses gmail (or one of the other providers, although they are fading) that google (or the other provider) can "read your email" -- as in they can read the part you sent to their user, and the replies their user sent back to you. Making it hard to fully avoid them being able to read some subset of your emails.