Lots of people gave great advice. Let me sum a bit of that up.
Flatpak
No error means success. You might want to install gnome-software-plugin-flatpak to make it available in gnome-software. I’d advise against using flatpaks if you also have the software available in the debian repositories. Always use the package manager instead, when possible. Those packages are maintained by the debian community and tied into the rest of the system. Flatpaks are not.
sudo
What would you like? That is kinda intended behaviour and an integral part of security. But you can have it remember the password for some time. Or ask for a different password.
FDE
I think a clean install is the quickest way to fix this. You can also get the other things right while at it.
DE
You can choose the desktop environment while installing Linux. You chose the default Gnome Desktop. It’s designed more like Apple or Android tablets. I quite like it. You don’t have that menu blocking space on the monitor. Instead you just bump your mouse to that top left corner or press the windows key and you have a fullscreen menu with your favorites. You can also directly start typing the first letters of the application you want and hit enter and start is, without lifting your fingers from the keyboard and it’s way quicker and more streamlined than clicking on things in a windows start menu. You can have an addidional dock somewhere at the left or the bottom with an additional gnome extension like this or what other people suggested. But if you’d like it like windows, why don’t you try the KDE destop? I think you’d be more comfortable with that if you’re looking for something alike the windows experience.
Ubuntu
Ubuntu regularly does some stupid things. I’ve been using Debian for quite some time. I suppose the differences for you are minor anyways and you could have it look the same if you found out which gnome quick-launch bar to install or if you used the KDE desktop instead.
Rozauhtuno
jvrava9