Probably a Surface (if there’s one with a 5G modem) at the moment. I got a Lenovo Ideapad Duo Chromebook a few years ago but it’s become unusable now and hasn’t been jailbroken to my knowledge. There was a newer version but it wasn’t made available in the UK. Devices like the Juno look promising but are way too expensive for me (accepting they’re around the same price as an iPad and that Android devices are subsidised).
K-9 is being used as the base for Thunderbird for Android but it isn’t there yet. FairEmail is a lot closer in functionality at the moment.
I use Thunderbird for Gmail (among other accounts) and it has to regularly compress my Gmail box, which none of the Android clients do - in my experience, Aqua Mail, K-9 and FairEmail all struggle with a decent sized Gmail mailbox after a while.