I get your point, but a) you still need Spanish provinces to form Andalusia, and b) Iberian provinces are way richer than the Maghreb, so it will still make more sense to expand further into Spain (also for fully controlling the Sevilla node). Also re-Islamicizing (?) Spain shouldn't be easy even after Castile is defeated for good.
It's not really about re-islamicizing - Humanist Muslims with loyal Dhimmi have next to no incentive to convert any heathens anyway - so much as it is about the Arabic alternative for Spain being... well, an alternative to Spain. As it stands now, there is a distinct
shift if you choose to form Andalusia as Morocco, say. You're shifting your focus to Spain, and the culture flip reflects that. If you make a Maghrebi Andalusian culture, what you're essentially doing is making Andalusia the go-to formable for Maghrebi nations for ideas and a game goal, which I don't necessarily think is accurate.
This specific issue has a pretty easy solution, though. Unmoor Andalusian from the melting pot events creating Mexican and Platinean, and instead give them alternative melting pot cultures to form.