That’s… it? You can get knighted for being “fairly good” at your job for half a decade, and then quitting?
Yes. Knighthood is generally up to the whims of the monarch. Although to make it there, it’s generally expected you have an achievement significant enough to be befitting of one.
But from what I recall, there’s little stopping his majesty from conferring a knighthood onto Chief Mouser Larry for his research into the napping suitability of 10 Downing Street’s furniture, if he wanted to do that.