Can you add ZOFE compatibility via
is_playable_country = yes?

Apologies for not specifying, but you answered my question with your response. I'll have to give it a try sometime, the switching is very annoying and I like that your mod goes further as you described.

The AI will only use the policy default policy which closely matches what vanilla does so there shouldn't be any balance changes from that.

@Baka Racker Sorry, do you mean parity to overlord or from a protectorate to a vassal? If you mean protectorate to vassal, it will be the same as base game.

If you leave it with the default policy, it uses the same research bonus and at the same tech ratio that protectorates get in vanilla. With this mod, when a vassal falls behind in tech instead of dropping to protectorate they stay as vassals but get the protectorate boost (80%) at the same rate as the base game. When they reach about the same ratio level as defined in vanilla the bonus is removed.

So at a minimum this mod prevents the switching of subjects between vassals and protectorates that can be irritating while preventing the vassal from falling outside of the tech ratio level vanilla expects. The other mods I saw on the workshop just disable the switching but don't account for what the result of that would be.

I like this mod, but it almost sounds too strong. When testing it, how fast do vassals reach a rough parity versus in vanlila?