I use uBlock Origin, HTTPs Everywhere, EFF's Privacy Badger, and Canvas Blocker on Firefox, but I also have Firefox set to clear all my history, cookies, and other browsing data when I close it. I also use a router-based ad-blocking via Diversion (formerly AB-Solution) and pixelserv-tls as well as dnscrypt via dnscrypt-proxy.
Obviously, Gmail is the last holdout.
My question is: with just my Gmail account, what can Google know about me or do to me? My only concerns with leaving Gmail for something like FastMail or MailFence are the potential reliability issues (both in sending email reliably from my own domain and accessing it as often as I do now). It'd be nice to have both end-to-end encryption, and encryption, but given how powerful a $5 wrench is, it's probably wishful thinking.
(For completeness: I stopped having a Facebook account. I log into my LinkedIn account once a year. I log into my Twitter under five times a year. I don't have the mobile apps (first-party or third-party versions) with any of them. I don't have a tinfoil hat, but I have bought https://www.amazon.com/dp/B01I497JAM/ for someone before.)
You should ask yourself: Can you get into your Steam library or Amazon account if you can't get into your email? Do you have a workaround if Gmail deletes your account? (If you have your own domain you can redirect, you do. If you use @gmail, you don't.)
It's exceedingly unlikely Google would do this, but bear in mind, the steward of your email account could reset the passwords and/or grant themselves access to any data you have on almost any digital account... and in many cases, hide the evidence they did so.
With regards to reliability, FastMail does occasionally have very short outages for technical difficulties which you can see on their Twitter account, but as someone who lives out of their email account so much that I browse social media largely by mass subscribing email notifications to a folder, I've only once tried to get to my email and been briefly unable to in the time I've had FastMail (over two years, I believe).
There is no way to opt out of this, the only way to remove this data from this form is to delete the email. While Google claims they no longer use your Gmail data for ad tailoring, they arguably could use the purchase history data they store about you... much of which comes from your Gmail data.
email metadata analysis https://labs.rs/en/metadata/