There’s no reason that can’t all be resolved in IX.
Certainly valid, and the question of Rey’s past is one mystery that probably won’t be fully realized until IX. But saving all of it for IX might make that film a little too dense considering it also has to wrap up all the events of the trilogy as well. But certain things like the lightsaber and Maz’s involvement feel like they could lose relevance to the overall story if they aren’t addressed soon, which would turn the conveniences of one film into conveniences of the trilogy. Just my opinion though.