“Assez” conveys the idea of ‘enough’ but it can also be used interchangeably with “plutôt” when “plutôt” is used in the sense of ‘rather’.

“Plutôt” conveys the idea of ‘rather’ as well as ‘instead’ ..

Hint: Look up “plutôt que” … plutôt que de faire quelque chose –> instead of doing sthg

Check out the Larousse Chambers dictionary entry, that should clarify it.
Larousse Dictionaire