I picked the knights because I think the character's belief in knighthood as a goal is quite central to her. What's the point of seeking it if we don't believe in it?
Oaths are central to that as well. I guess the Vale Knights' response is a bit more tailor suited for chivalry etc, but as a Queen regnant, oaths will be a very direct and practical concern to us - and yet the notion of faithfulness still touches on Rhaenyra's idealism and chivalry.
Where Knights from other Kingdoms mostly become such for the sake of personal glory, in the Vale there is the duty of protecting Smallfolk from Mountain clans raids.
That pre-assumes a level of choice. True in our case, but that's a very special case. As it is, people become knights mostly because it is expected of them. After all, it's not just a job or status, it is part of how you grow up and are raised, as pages and squires and so on. And that a way of being raised that will just be expected of most of nobility, certainly for all sons of higher nobility and at least the heirs among lower nobility.
Also, speaking of other kingdoms, I am surprised Tarly didn't speak up: Dornish raids into the Marches are not that infrequent, so at least Marcher knights would have that in common with Vale Knights (plus of course the Marcher raids into Dorne, for that matter). So it isn't
just the Vale. And even everywhere else, you'll get at least a regional conflict every generation or so, though at least not all-out wars; the Targaryens have been pretty good in limiting those to every century or so.
This is pretty much fighting against windmills. The Mountain Clans lack sympathetic viewpoint characters, so the Vale knights nearly always get a pass from the fandom for being enforcers of a genocidal settler policy that has been ongoing just as long as the Freeholds antics.
I mean, that history is also part of it. The
Mountain Clans have that name for a reason. That genocidal settler policy in the Vale proper by and large happened 4,000 or 6,000 years ago, depending on where you want to set the Andal Invasion. Of course, I have no doubt that Clanners will still be slaughtered by Vale Knights wherever they can get a hold of them, but it just isn't the current crop of Knights which did the genocidal replacement population thing and it hasn't been for not only centuries, but indeed literal millennia. The current Knights are 'merely' upholding a status quo.