To answer questions 1, 2, and 3:
Yes you can 👍
yep, do your rice. Maybe visit the Unixporn community.
For the most part (in MY experience) changing DEs only involves installing the desktop package, choosing your login manager, and then removing the old DE and its native apps (unless you want to keep them)
It's very simple to have multiple desktop environments installed. You can select which one to use from the login screen, or launch X from a console and then start the DE from the command line.
It would be no problem to remove the office software you're not using. I doubt that's connected to any other packages through dependencies.
I could use flatpak to get the newest version, but could I then get rid of the pre installed old version?
Of course you can and if you decide to go down this route you should take a look at Flatseal that can help you with flatpak permissions and theming.
debian's default graphics look also prehistoric. Can I change that installing other styles?
Debian installs Desktop Environments without any theming, just plain vanilla DEs and in most cases, expecially with XFCE, they are not that pretty out of the box. You can still theme it to look and work the way you prefer.
with debian you are asked to choose the environment: xfce, mate... how troublesome is to change those after installation?
sudo apt install new-desktop-environment, log out and log back in selecting a different desktop in the display manager: piece of cake
This doesn't mean that xubuntu has bloatware, but simply much more pre installed packages, right?
Keep in mind that the 0.7gb debian image is the "netinstall", which pulls software from repository instead of installing it directly. There are also debian "DVD" images that can be up to 4gb as well. And yes, Xubuntu has more bloatware but not as much as you might believe
Why do you not switch from Xubuntu to Linux Mint ? Mint is a derivate of Ubuntu but without Snap.
with debian you are asked to choose the environment: xfce, mate... how troublesome is to change those after installation?
I just install whichever ones look interesting, and pick the one I want to use, today, from the drop down, on the login screen.
Yes, Xfce is lightweight; which may be seem crude.
If you select Mate or Wayland/Plasma then you would probably enjoy it more visually.
Once installed, you can select a new desktop environment at login. There's generally a dropdown or menu item that would list all available.
With those other desktop env., you can say goodbye to that 0.7GB, though.
I tried several other desktops on debian before settling on xfce. Never had a problem switching between them. A choice of a dozen or so gtk themes is available (try "apt list | grep theme") including adwaita and greybird.
I'm not sure, but i wouldn't be surprised if they have a dpkg-reconfigure script for window manager (desktop environment) switching.
But even if not, it's no trouble at all. You can have multiple DE at the same time. Your display manager (login mask) should let you switch between them.
And yeah, packages are named gtk--theme in Arch, i think it was similiar in Debian. Then change via xfce4-appearance-settings ("Appearance" in menu). Or just download and drop a theme in ~/.local/share/themes, as long as it is a gtk-theme. Though for window borders, it needs a folder "xfwm4" in it.
Please learn the basics of APT
The installer installs a ton of bloat, but of course you can uninstall everything you want.
Thats what I usually do on Debian.
Debian stable will always prioritize stability and provide you older versions of applications. Even Debian Sid(their testing/rolling release version) gives you less than bleeding edge versions of apps. You can always install your own versions by downloading from provider or building yourself but if you're wanting more current software, I'd consider another flavor of linux.
You can always install other themes, icons, etc. to get the look you want, Debian is just the underpinnings of the desktop. Using XFCE there is no different than using it in another distro.
The size difference is because of preinstalled applications, as you suspected. Some call it bloat, others just understand that Ubuntu is trying to cater to "set it and forget it" user.
Not sure what you're on about... Sid and testing are usually pretty damn near bleeding edge.
While this is generally true, there are sometimes exceptions to this, especially during the freeze.
Even unstable slows down during the freeze, as the usual way to get stuff into testing is through unstable, and packagers, especially of large collections or important dependencies, opt not to disturb the freeze by dumping a bunch of bleeding edge stuff in there. Consequently you also get more new stuff in unstable shortly after a new Debian release.
I mean sure, but that's a period of like a couple months every couple of years.
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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