Accept that there’s going to be political diversity and social change (for better and worse), and try to create an overarching framework to channel it into something other than violent conflict. One idea:

Let societies do whatever they want, but institute a “risk mitigation’ tax (or other form of resource redistribution) based on size and similarity: if a social strategy is popular and widely adopted, it’s taxed at a marginally increasing rate until it reaches an equilibrium level; and the revenue is used to fund more experimental social strategies. This flips the historical dynamic on its head: instead of each society trying to forcibly convert the rest of the world to its own system, each society has an interest in discouraging others from following its example.