If the point is to get away from large, data hungry corps than going to Amazon would be a funny choice.
If you host your email with Amazon WorkMail they have access to all your emails just like Gmail or G Suite does.
As you say, they are not dependent on their ad business, but they are making a lot of money from it. Expanding operations to improve profits is a no-brainer and they can existing distribution networks like AWS to have an extra edge.
[0 ]https://digiday.com/marketing/amazon-ad-revenue-2-2b-132-per...
Your point is moot, sorry