Aedunia in Gaul is my recommendation for playing as a tribe. They get a good unique deity and can easily eat all of their neighbors, and Gaul is absolutely massive once united, with just the claims from forming Gaul being enough for great power status. The most important thing to keep in mind when playing as a tribe is that you want to reform as soon as possible you can gaining tech levels and building cities, ideally within your first ruler's lifetime because you need 80+ popularity to reform. The recipe for success in all of Britain/Hispania/Gaul is to ally two of your neighbors that you don't want to immediately annex, save up at the start until you can hire a small merc stack, and then chain wars forever, you+two allies+mercs will easy win every war. AE and Happiness are mostly non-factors for tribes because everybody around them is the same culture group and religion (except for iberic religion tribes), but stability does need to be at some level (40?) to reform to a monarchy/tribe.
I always go monarchy but it's up to you, republic mechanics are more interesting but can be annoying.