Because I'm not a conquering power, I'm using type IV governments in all settlements bar Massalia. I'm playing on M/M. That's probably why the Aedui and Arverni have largely ignored me so far. Though the Romans keep trying to bribe the governor of Tolosa, having given up on doing the same to my faction heir in Massalia.

Raising an entirely Celtic force to have a go at Burdigala sounds like a plan, though in a bit when my economy can support one. Does Burdigala have mines? More rebel settlements is good since it doesn't provoke an immediate war. Storming down the east coast of Hispania isn't going to happen, though. Its one settlement at a time at a gentle pace, building a confederation takes time.

I could "exile" a family member to Krete. Ensure that him and his whole branch of the family is removed to there, then send no support once they're there. Kyrene is another possiblity, though that would put me right between Egypt and Qarthadast. Fun later on in the game, mind.