these are data control methods for encoder, cbr spams same amount of data every second no matter whats on the screen, not very good for recording. crf tries to keep same level of quality depending whats on the screen and adapts bitrate smart way.
you always need vsync, but some games may record better or worse without it
no downsides, encoding will be done by intel gpu freeing cpu load.
https://obsproject.com/forum/resources/how-to-use-quicksync.82/
some1 could argue x264 always has best quality but diff is nearly nonexistent unless you are fanatic and analyse image quality with magnifying glass. qs may result in slightly bigger files
when you enable it, win10 will probably install some old driver automatically. run DDU in safe mode after, clean intel and install latest stable from intel site. installing new over old without ddu will 99% lock you in crash loop