One of the frustrations about Google’s tracking is that it’s unclear exactly how much data they are collecting and how exactly they are using the data to track. You seem to be assuming that Google’s tracking is purely interest-based. However, we know that targeted advertising goes beyond interests. Medical conditions and financial status can certainly be used for targeted advertising. And while nearly everyone may visit the doctor and have a bank account, not everyone has the same medical or financial portfolio. Google does claim that they no longer use the content of your emails to track you (they used to though, and there is no technical reason they can’t still do so, we just have to take their word). Even so, email metadata contains a ton of information that can be used for tracking. To take a healthcare example: you just started receiving emails from a reproductive specialist? Even without reading those emails, Google knows you are likely pregnant or trying to conceive. If the subject line is included in the metadata Google uses, they have access to even more information. Your bank has sent you an email with “late payment” in the subject line? Google knows your credit score is about to take a dip.
I don’t want to discredit the real security benefits using Google has, the company has an excellent security track record. But given your threat model of trying to prevent targeted advertising-based tracking, I think even using a Gmail account for “mundane” activities gives up more information than you may realize. And while Google’s security record is good, I’m not convinced it is so much better than Proton’s (though I lack the expertise to make the comparison in any serious way).