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