Commenting from my GrapheneOS Pix6, I actively prefer GOS to stock, and get a sense of disdain or my soul sighing every time I pick up my stock rom Pix6 now.
I bought my girlfriend a Pixel 6A as birthday gift last year and whenever I use it I'm blown away by how smooth and fun everything feels on GOS. Every other Android I use feels so sluggish, blown up and hard to use in comparison
I was explaining today to a close friend that I (anecdotally) have noticed a significant reduction in battery usage on my GOS pix 6.
The whole device feels snappier, more responsive, and I can certainly attest that I got 9+hrs out of this thing at max brightness playing terraria. Can't say the same for stock rom in the same conditions, while I don't have the technical knowledge to prove it (and I'm happy to be proven wrong) I'm convinced the majority of my stock rom Pix's battery is eaten by proprietary software phoning home.
With that said, unless I go out of my way to disable certain privacy aspects of this phone or implement spmit-tunneling on the VPN it's set to go through, unfortunately many sites/apps break. For these instances I generally use the stock pix. (Eg. Gov services/KDE Connect).
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.
Could you explain more about how you're using GPS whilst it is maintained in a different profile? How does GPS interact across profiles (which I assume is a prerequisite to use Google's email addresses on one's mobile)?