I think it would have been difficult to focus too heavily on integration with other tribes and maintain the narrative. There are a number of clues in the last book that Isherwood's tribe has comingled with other tribes while he was in his dotage.
I let the idea of this roll about in my mind over the last few days and I think you're right. Like you said, they do integrate with a nearby tribe at one point. And not that much is said about it because in reality the union of two peaceful tribes would likely be uneventful. In the event of there being a clash with something more threatening, you got a taste of that with the introduction of Charlie. It allowed for a perspective of how they would handle hostile outsiders without the need to divert the narrative to inter-tribe politics etc.