So I think Du Weldenvarden is a good, choice. Thats where I'd put it. For several reasons, one its in Alegasia, two its heavily protected so its a safe place, and three its close to the elves who are the most dedicated to the preservation of the dragons.
But Vroengard would work too, once Doru Areba has been rebuilt and all the radiation removed.
I just do not think that Du Weldenvarden should be dismissed, solely due to influence by elves.
But I also think there should be multiple Rider Strongholds. So no one can be over thrown.
There are also Beirland, Illium, Uden and Parlim as well. Those could be locations for Rider Outposts or Strongholds.