Why, first I would want a simple mod that gives Rogues a sneak attack / backstab during the reaction when they already have sneak attacked / backstabbed during their turn.

Thats according to the original rules, by the way, which is why I would want that as a separate mod.

Namely you can backstab once per turn - that means your very own turn, and during the reaction somebody elses turn.

In fact if you could get two reactions, which in BG3 you cannot, you could backstab on both reactions, as long as it would be a different victim each time.

Unfortunately many people seem to very consistently confuse turns and rounds, and what BG3 has is once per round, which is incorrect.

Rogues already got nerfed massively compared to the D&D3 version by only being able to sneak attack / backstab once per turn. There was no limit in D&D3 on how often you could sneak attack / backstab.