I had a similar situation. I've conquered Italy, islands, Massilia and Emporion in the West and Illiria with core Epirote territories in the East. I've started the war against the Lusos and in a short time all my northern border was a warzone as I was attacked by Aedui and Sweboz. At the beginning it was tough as the Lusos controlled all of Iberia and I had to bring significant forces there, at the same time maintaining defenses everywhere else.

Several times the Aedui were close to capturing Massilia and Epirotes managed to take back one minor town in Illiria, but that's all they've managed to do. The economy was strained at the time, but it didn't matter as I had enough reserves. Now, the Lusos are dead, Aedui lost 3 cities and I've managed to push Sweboz out of Alps.

I used Type IV government in each key region. It was Emporion in Iberia and Aventicos against the Sweboz (Massilia was easy enough to reinforce from Italy). In Emporion I trained Ilergeti soldiers and they fought together with the legions I've brought from Italy. In Aventicos I've trained Alpic phalanx and Helveti phalanx. They were more than capable of pushing the Sweboz back with some Celtic light troops from Mediolanum and a few legion units from Northern Italy.