Great Blue

Back then I was deciding between Posteo and mailbox.
I choose mailbox because, at least at the time, they had unlimited calendars and Posteo only offered 3 free ones.
Also the website of mailbox seemed more respectable / unspectacular / boring than Posteo. Posteos website got me some typical start-up vibes, means hoping for fast growth and quick exit. But there I guess I was wrong.

mailbox.org has a web app, so no need for a client app.
And Posteo also supports POP3, IMAP and SMTP, so you can use every mail client with support for it.

I don’t know how MacOS time machine works exactly, but if it constantly writes on the SD card you should consider changing to an external SSD or HDD. The best backup isn’t helpful if your backup medium dies.

Shredding might not work the way you expect on a SD card.

The memory cells in a SD Card can only handle a limited amount of write operations. A SD card typically has more cells than needed, so the controller can switch through different cells to improve the overall lifetime of the card. Which means you can’t be sure which cells gets rewritten when shredding, so the data you want to be gone, could still be readable.

If you want to secure your data, use strong encryption. Because what you gonna do, if you can’t destroy or get rid of the SD card?

As already mentioned for PC Mozilla Thunderbird.
For Android you could take a look at K-9 Mail. For Apple I have no idea

Or look into FreeTube, it works really well on desktop

And there are Android derivatives which are Google / PlayStore or Play Services free. Like Lineage OS, GrapheneOS, CalyxOS or /e/OS