

No idea about macOS, but this is something the typical Windows user should notice when switching over to Linux. That is, Windows OOBE gives you a user with administrative privileges by default and therefore won’t prompt you for the password again after logging in, just yes/no dialogs when exercising those admin privileges.
Typing in the password whenever you need root privileges is just part of the security model of Linux and unless for some reason you’re using sudo for everything, people get used to it. Your default user account doesn’t automatically have root privileges, sudo or su mediates that for you. Back when I used Windows, I even had my accounts set up that way, separate admin, daily user account without admin privileges, and prompt for the admin password every time I installed stuff, etc.
Granted, it does leave me with a couple compromises like a login password that is shorter than my disk encryption password so I’m not asked for the full thing every time I sudo and sometimes leaving a terminal with sudo -i hanging around.
Certainly. I’ve had setups with FVWM as a pure window manager while using XFCE’s
xfce4-terminal, MATE’s Caja file browser, and GNOME’s Evolution mail client. Some utilities will pull a few extra dependencies from their native DE, but they won’t get in the way either.Display manager won’t matter too much, most should be configurable to point at your WM of choice. LightDM integrates nicely with GTK themes, SDDM for Qt, and GDM for GNOME.
The biggest pain point from my experience was configuring power management and lid close actions manually, if using a laptop, since those often are only done for you if you install an entire DE at once.
Also grab a copy of qt5ct if you’re interested in making your Qt packages look more integrated next to GTK packages.