Septimania actually was conquered by the Umayyads in around 719, but it didn't last more than 35 years.
Worth noting that Septimania is not really France at this point; it was an appendage of the Visigothic Kingdom. While the nobles there railed against the Muslims, they were also anxious about the Franks, and only acknowledged Pepin as their suzerain when he promised to uphold the Gothic law if they supported him kicking the Muslims out of their cities. A lot of the pro-Frankish sentiment among the Gothic resistance was led by Count Ansemund of Nimes, who had outsized influence among the Gothic counts and led several of them to throw their support to the Franks. A good start might be killing off Ansemund and replacing him with a Count of Nimes who's more anti-Frankish, and have the Moors leverage that fear of the Franks into a tacit alliance with the stronger Gothic lords to keep Pepin out of their domains. That might buy you enough time for Muslim Septimania to at least build up a foundation, but it will always remain a vulnerable appendage of Al-Andalus.
You could always buy them more opportunity from there by having Charlemagne die before he can really do anything (or just have him grow up to be Charles the Mediocre, for that matter), or adding a successful Aquitanian Revolt to weaken Frankish power in the south.