A (small) part of not putting all your eggs in one basket is also avoiding vender lock-in. Having your personal email with proton, and your password manager with them makes it very difficult to switch in the future if you need to.
On a side note, I use anonaddy (now Addy.io). It allows you to create email aliases on the fly. So when I sign up for a new account somewhere, I generally make up some email like "[email protected]" for the email and save that right to bitwarden.