5.15. isn’t that bad of a kernel version in my experience. Admittedly, I’m don’t have any latest gen hardware at the moment, but using one generation back RX 6700XT without problems on it with Mint. Alternatively, one can install the newer 6.x kernels with a few clicks if needed, they are not actively blocked or unlisted.
Switched from the default win10 mail app to thunderbird about a year ago when the mail app started forcibly updating to the outlook and broke some shit on my windows installation to use a whole lot of resources. I quite liked the old mail app of the windows, but Thunderbird is quite enough of a replacement at default settings and much more customizable after fiddling. K9 has no difference than Gmail on default settings, either.