Forming bigger nations from various subjects

Hey there,

i'm in a japan run and step by step vassalising the indonesioan nations. It's a big old mess. I always wondered what's the most infamy efficient way to combine various subjects to larger nations. Mainly for aesthetics, not min/maxing.

I know that you can conquer a state and release e.g. Ethiopia and feed it the other states in the region, starting investment right wars and adding the transfer state wargoal.

Is there a better way? What's the best approach to form e.g. indonesia when i already have several subjects in the region.