Perhaps Charlemagne was attacked by the Basques as he entered Iberia rather than as he was leaving? The loss of so many experienced soldiers and the fear that further losses might weaken his eastern frontier deterred him from pressing further.
I think that would be better, having Basque, Navarro-Aragonese and Asturian states engaged in the reconquista, I think Having a Pyrenees border to Western Francia will be better instead of an Ebro Border that I think would make Spain more unified.