K-9 Mail for Android has merged with Mozilla and will eventually be renamed to Thunderbird. Its UI has seen a lot of improvement these past couple of years, and the backend has always been reliable for IMAP (including push notifications).

Sorry for the delayed response here, however I primarily use a proton address, and I currently have a redirect in place for my Gmail to the proton. My intention is to close the Gmail all together, however that's not yet possible as I will likely miss important emails in the process, I am (as discovering) updating my email addresses for each service as it becomes known so as to avoid such occurrences.

As it stands, I have GPS on another user profile to add to the security provided by GOS sandboxing, not that I don't trust GOS devs, I don't trust GPS not to sneak in somehow.

I hope this answers your query, do dm me if need be for further explanation.

Edit: I cannot speak further as to email clients, as I have yet to perform further experimentation. I do intend to get to it soon, though if you beat me to it, do message me to let me know how you went and what you'd do differently.

Edit pt2.: I have checked, both my pix6's were from a close batch and manufactured the same month, I suspect due to this battery degradation is not applicable.