I'm not 100% clear on what you have left for the players to do in your campaign, but I do wonder if giving your players a choice at the end of the campaign is the best way to round things off. Ideally, give them one or more situations to resolve, like a classic BBEG to defeat, loose ends they created that need resolution before they can rest, and so on. But leave the "this is how it ends" up to the players. Ultimately, the campaign should end when there are no useful threads left to play out - it might end happily ever after, it might end in tragedy, but either way, it should end with Answers, not Questions.
For example if the King wants them (or some of them) to be Knights, why is that? "As a reward" isn't a good reason, for the players - the King must have his reasons, then the players can decide how to react, which may or may not be the ending of the campaign. For example, maybe the King wants to make sure the PC's are kept close, to make sure his realm (and rule) is save for ever more. Makes sense, so what might he do when some or all of the PC's refuse? Maybe it's not the end of the campaign, after all...
For example if the King wants them (or some of them) to be Knights, why is that? "As a reward" isn't a good reason, for the players - the King must have his reasons, then the players can decide how to react, which may or may not be the ending of the campaign. For example, maybe the King wants to make sure the PC's are kept close, to make sure his realm (and rule) is save for ever more. Makes sense, so what might he do when some or all of the PC's refuse? Maybe it's not the end of the campaign, after all...