There’s hotkeys for workspace navigation, I use them all the time: Super+Alt+Left/Right will navigate to the next/previous workspace (Super+PageDown/Up also works). If you go in Settings -> Keyboard shortcuts -> Navigation you’ll find more, notably to move windows between workspaces.

As for the fullscreen by default, there’s extensions for that I guess, though personally it doesn’t bother me since after the window is maximized once it should restore its maximized state the next time it’s opened. With Super+Arrow keys to position the window, I personally barely use the mouse for window management.

The Nextcloud AIO container itself doesn’t have access to the backup directory, but it has access to the docker socket (/var/run/docker.sock). Having access to the docker socket means it can perform any docker operation on the host system, in this case starting a separate backup container with the backup directory mounted.