If Aegon V’s kids went through with their arranged marriages, how would that have changed things? Would he have been more successful with his reforms? Even if the great lords went along with the reforms, there would still be widespread discontent with lower lords losing rights. Would there be a great threat of many lords turning to the Blackfyre cause?

Given that there was widespread discontent without the support of the Great Houses in OTL, it’s pretty much axiomatic that the situation would be improved in this timeline – fewer political opponents, more political allies, an improved reputation, and militarily, your potentially rebellious lord is going to think twice about bucking a king who had the Baratheons, Tyrells, Tullys, and Redwynes backing their play, if only for purely pragmatic reasons.