It would be a faster and better way to do that than creating functions in every save directory which just run one command giving a player the next stage.
It could look like this:

"rewards" : { "unlock_stage":"stage_name" }

It looks like advancements are hard coded to only allow exp, loot, recipes, and functions as rewards. I would recommend using the function reward type and running commands from the mod instead. This is how modpacks like SevTech Ages have accomplished stages as rewards for advancements.

I will look into adding this, however SevTech has already managed to do this.