I have been slowly degoogling my life by reducing my use of Google apps and switching to FOSS alternatives. I recently created a Proton account and tried to keep it completely isolated from my default Google account so as to not let it know what my Proton mail identity is. I have finished my transition in my desktop to the best of my ability and am planning to start the same with my mobile. The only thing I have done in my Android phone related to Proton is having downloaded ProtonVPN and signed into my account within the app. However I fear by downloading ProtonMail and signing into it, my Android phone would identify the email account, which could make this entire transition useless. Is there a way to avoid this situation or has the damage already been done. What does Google know about my Proton account, if at all?
I dont think you can. As long as you use Android with GMS, Google will always know. Best is to get yourself a Pixel, immediately flash GrapheneOS, and use it.
Basically pointless to hide from Google on OEM android, but tutanota, Molly, and im sure other apps, will at least not send message contents as a notification.