iDrive E2 is $40 a year for 1TB S3 compatible storage and they have promotions quite often. As always with cheap storage don’t rely on it and have a local NAS but it’s handy for offsite. I’ve just transferred out of Wasabi, who were cheap but are less so now.
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.