I have been self-hosting my mail server for the past 5 or 6 years with success. Recently my ISP decided to close port 25 so I have to use a third party to deliver my outgoing mail.

I genuinely don’t understand what you are paying for. I must have missed something.

What cost?

It is hard to set up and you might need an SMTP relay since most ISPs close port 25. But it is feasible.

What I do is a local backup on a different disk with BorgBackup, then a copy of that local backup to a Pi at a friend’s place, with rsync.

You won’t find apps that provide the same service as permitted by the entire Google ecosystem. That would mean that they are as intrusive as Google and you wouldn’t be willing to use them. You either choose convenience and you can keep Google or renounce it a bit and use one app per function.

Upgraded to Debian Trixie two days ago. Runs flawlessly

I second that advice. When choosing the HDD, make sure it is NAS-grade.