Linux

Da es verschiedene Arten/Sorten von Linux gibt und ich auch hier auf Mastodon immer wieder was von #Linuxmint gelesen habe, wäre Linuxmint eine gute Wahl und warum bzw. warum nicht?

Linux Mint is what I recommend people use as well. The desktop UI (Cinnamon) feels familiar coming from Windows, and the OS is designed to 'just work' for the general user.

Wie sieht es mit der Kompatibilität von Dokumenten aus, die sich auf Sticks und Festplatten befinden (Stichworte: Formatierung, Windows)

If you mean the documents themselves, Mint's built in programs will open any reasonable file you throw at it: documents, images, etc. You might need to install a program if you want to open something exotic, but I have had no issues.

If you mean what the flash drive filesystem needs to be formatted as, exFAT, FAT32, or NTFS will all work on both Mint and Windows. I prefer exFAT as it has the least limitations and works everywhere natively.

Wenn ich zunächst Linux nur auf einem Stick installieren will, wie groß sollte dieser sein und was sollte ich beachten, wenn ich zusätzliche Programme (Office-Paket) installieren will?

Running Linux off a flash drive, ie as a live environment, works well with a 16GB or 32GB+ flash drive. Do note it will be slower than running it off an SSD, but it is great for getting a feel for the OS and seeing if it works for you. You can install and do anything you could as if you had installed the OS to your SSD. Just note, by default this will be a live environment, it only exists as long as the computer is running; no changes to the live environment persist through a reboot. However, that is just the default configuration, you can make the USB environment persistent if you wish.

You pointed out an office suite specifically: Mint actually has an office suite (LibreOffice) and basically every normal program already installed for you.

Good luck man, and I think Mint is a great place to start!