So, here we go...

A way to render NPCs only for specific players, either:
A) Based on a permission, or
B) Just on a simple list where you can add/remove X player by command.
C) or Both <3

/npc visibility change TYPE [types: all, per-player, permission]
/npc visibility add/remove NAME_OF_PLAYER
/npc visibility permission NAME_OF_PERMISSION

And in general, a way to edit properties by command.

Thanks for the feedback, like the idea.

Could you please clarify what kind of permissions you mean in this case ?
The normal permission levels like https://minecraft.fandom.com/wiki/Permission_level#Definition or something different ?