Earlier quoted context omitted.
You're correct in that Google doesn't discriminate between paying and non-paying customers: It happily abuses the privacy of it's paying customers too. Many of Google's uses of data are not specific to whether or not someone is using G Suite or consumer Gmail as far as I am aware.
What Gmail data is being abused?
I definitely don't want my email used to train Google's AI models, which might unpredictably cough up sensitive data since machine learning is pretty much a black box.
A big issue is that at any time, Google may introduce a new "feature" and decide to use your data to power it. I also think of how purchase data gets extracted from your receipts to store in your Google account profile and the like (https://myaccount.google.com/purchases although the text on this page has changed... did they stop?) and I feel a privacy-first service would need to at minimum have switches to control what kind of data processing is performed on their email, and default new features to be off.