in a terminal, type flatpak remotes. If it lists flathub, you’re good.
Try installing a random app like flatpak install flathub de.haeckerfelix.Shortwave
It shoud work. If it doesn’t, post your logs.
I have to type a password in the terminal every time I want to use sudo
This is the intended behavior and should not be changed, it’s a basic security feature. Once you’ve finished setting up you system, you shouldn’t need sudo everyday anyway, except for updating/upgrading the system.
I’m used to a desktop interface with a toolbar/start menu that I can pin frequently-used programs to, but with Debian it seems like I need to click “Activities” to do anything. Is there a way to set up the interface so it’s more like Windows in that regard?
Assuming you’re using Gnome, this is easy to solve using Extensions. First if it’s not installed already on Firefox, install Gnome Shell Integration. It’ll let you manage Gnome Extensions directly from extensions.gnome.org
Then, install dash-to-panel for a “windows-style” experience, or dash-to-dock for a “macos-style” experience.
After that, you can go wild on the extensions you want to use ;)
If I need to do a clean install, I’m thinking of switching to Ubuntu, since I’m more familiar with the interface.
Don’t. Ubuntu will teach you nothing but the Ubuntu way. Debian is as Standard Linux as conceivable. If your only concern is the Ubuntu-style interface, configuring dash to panel to appear on the left side is all you need.