AFAIK fast startup only affects shutdown, clicking restart will always do a full reboot. Shift clicking shutdown will do a full shutdown like you said, but shift clicking restart will start recovery mode.

I explain fast boot to people by saying "for some reason Microsoft went and made the Shut Down button not actually shut down your PC, it really just puts it into a 'deep sleep' mode, and to their credit, it lets them say that boot times are faster... But it also means that in order to FULLY restart the PC, you have to click restart... I know it's a pain"

Usually I get looked at like I'm from another planet, but that reaction means they'll probably remember it later.