Been using k9 for over 4 months now. Pretty nice. A couple of not so big deal issues, but overall pretty nice.
I wish it had material you design where it followed the system tint color. I wish it would stop complaining about authentication here and there. And last wish is to have the delete icon on the bottom instead of all the way at the top. Lol
Yes, I'm several years into my de-googling process and a solid email client is not something I'm worried about. K9 is great and, as Thunderbird, we can only hope that it gets better.
Look into installing a custom ROM, some phones are not supported but many are by at least one custom ROM. I personally installed GrapheneOS and recommend that or calyxOS (never used it but hear good things) but those are only available for pixels, iirc LineageOS has the widest support but also isn't quite as secure. F-droid and Aurora store for apps.
OnePlus phones are rootable if you buy them unlocked (not from a carrier), and you can install anything you want on them. Some of their older phones even support straight up Linux mobile.
They're different in some way I guess. The material design is basically Google's HIG, the you is the tinting from the wallpaper colors. Just like how KDE does their tint the whole UI from the wallpaper.