Instead of doing all that stuff and getting your hands dirty with AWS and .tar .gz etc., would you use a web-based email client where, with one login, you can access to all your email addresses?
Not in an unified inbox, but have them as separates. Say cmd+1 is quickly switching to address #1, cmd +2 is switching to address #2 and so on...
When you'd be switching devices, you'd just log in once and get access to all of them.
Would you use that?
Otherwise (with multi-logins), I could do that with my provider's email web UI in browser tabs (or save the logins in the browser, which I generally do not), for all the ones I designate to them as separate mailboxes (they have some number included, then you pay more after that), or could have a mutt script+window to switch to a different mutt data directory for each.
But I have the other backup stuff largely automated now, so I don't mind much. (Plus it makes me feel good to run scripts I wrote & tested carefully, on my PC.) There would certainly be simpler ways. Maybe I'll reconsider someday if I want to change. :) (Edit: for my own data, I avoid SaaS generally, as I like a sense of control. Same reason I guess that I go to the trouble to learn/use OpenBSD.)