When it rains, sanity goes down, that's fine, but you could make an algorithm that looks like this:
If it rains and the player is in the rain in the dark, sanity drops a lot,
If it rains and the player is in the rain but there is light, sanity will decrease slightly,
If it rains and the player does not have armor, sanity will not go down,
If it rains and the player is not in the rain, sanity will not decrease,
If it's raining and the player is not in the rain and is next to a heat source like a bonfire, sanity could go up.
I say because I like the rain and I think it should sway a little.
Good mod, take your time