Is it possible to make clans move to a stronger tribe even if their relations are not very good, but rather neutral? And if so, then you can go from the opposite, implementing a mechanic in which the clan could refuse to join, despite the cordial relationship, if the tribe does not have enough strength to keep it. In addition, I think it could look like a scale of relations, on which there are certain zones of "possible diplomatic actions": depending on the characteristics of the tribe, these zones could move, change their size or disappear altogether.
Viewing post in Fixing Bugs and Adding Decisions comments
The decision to change allegiance will depend solely on the relationship strength to keep things simple and easy to balance for the next version. Later I'll start adding more parameters that take into account relative strength, etc. Though right now I don't have a metric to measure a clan or tribe's strength.