Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Lack of mod documentation frustrating

I've added a page on how the Treasure system works: https://docs.larian.game/Treasure_Tables
I hope you'll find it helpful.

In practice, you'll need to either create new Treasure Tables, or mod the existing ones, to drop only high-Rarity items, and ensure the traders use those instead.