In one point I give you right... The AI should prefer the own culture and/or region in marriages. This is no question I think. This is logical.
But it should be still possible to marry someone at the other end of the map. This is my opinion. There were always exceptions.
But dynasty relations sound nice yes.
It should be possible because there needs to be room for some ahistorical options but this should be more for the player than the AI.