> I don't want to do it locally, since that puts me on the hook for assuring physical safety and backups of my backups.
Regardless of where you store your personal data, the proverbial buck always stops at your desk when it comes to assuming responsibility.
Paying someone else to store your data gives you someone to hold accountable to what happens to your data. But that accountability is always limited and doesn't dismiss you from managing that relationship. For instance, the other party is still entirely free to bring terms and conditions to the table, which are meant to mitigate liablity as far as the party which will host your content is concerned.
Put more succinctly, even when you rely on a cloud service, you're still on the hook to assure safety and backups of your own data.
> I suppose I could roll my own solution, but frankly I get tired of maintaining my one-off projects like that after a year or two.
Yeah, I'm gonna prod you on that one. :-)
For e-mail, you could run an script once a month that harvest all your e-mails over IMAP and roll them into TXT, HTML, EML,... whatever. You could go for one big file, or discrete files. You could even pain yourself and fit everything in an mbox file. Roll everything in a ZIP or TAR ball. Then you'd store output file in whatever service you want.
You could spin up a VPS, provision the entire thing and set up a script that does all that as some service or cron job or whathaveyou. Now you've got an entire stack to take care off. Not what you want.
Maybe this is a good use case to go serverless. You could string something together using AWS serverless. Write an AWS Lambda in Python or Java. Should be one single script. Store your e-mail backups in S3 or somewhere else.
The idea of such a solution is that it just runs in the background and it runs in a robust fashion with not too many moving parts. So, here you have something running indefinitely as long as the bill gets paid. Or amazon doesn't cut you off. But at least you've now got your e-mail mirrored on two independent services, which is marginally better then where you are right now. The only other major constraint here is being able to restore your e-mail from that backup in case of disaster.