I've been considering doing a similar thing but I'm in a different situation with Gmail. I hopped on to their business domains when they were initially released (and free) and I'm still grandfathered into their free plan.

I use the account as my primary Google account which includes purchased Android apps, and I make pretty heavy use of that Google Drive with several external organizations.

I'd like to move just my email away to my own hosting, but I'm worried Google might do something sneaky when they realize my MX domains no longer point at them.

I haven't herd of anyone attempting to migrate just their email from a Google on a custom domain while trying to maintain the rest of their account and that fear is the only thing keeping me on Google.