GreatBlue

PocketBook also has nice readers and as far as I know they run an embedded Linux as OS.

Basically, yes, they couldn't use them. The old 2FA had a really weird implementation...

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.

Meines Wissens steigen die nur kurz aufs Rad für die Presse oder unter hohen Sicherheitsvorkehrungen, die es für den Alltag nicht verhältnismäßig machen.

Generell gibt es Sicherheitsvorgaben für Minister, Bundeskanzler und Co und das nicht erst seit dem Ukrainekrieg.
Das beinhaltet u.a. kein Fahrrad- / Escooterfahren. Selbst öffentliche Verkehrsmittel wird schwierig. Mal eben ÖPNV ist nicht. Beim Bahnfahren muss meines Wissens mindestens der Wagon gesperrt sein. Daher nutzen die Minister und BK den Fahrdienst oder Flugbereitschaft. Ist für sie selber und alle anderen entspannter.

I have a FP4 and rooting was possible. I'm pretty sure they haven't removed the ability for the FP5

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.

I'm in the name after location and function fraction. All but my printer, he's named Cthulhu because printers are a menace to humanity and it supports wake-on-LAN.

In the end you would build your compiler directly in assembler, so no compiling would be needed.

But if you run your compiler on compromised hardware it would still be possible to insert a backdoor in your programs without you knowing.
To mitigate this vector you would be required to build your own chips... with self developed and assembled machines all the way down starting at growing your own silicon crystals.

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?