Today I played an old racing sim and I had the idea of this suggestion: yet another compound...
Two different rain compounds: Soft wet tyre and Hard wet tyre. You can choose if you have to be faster or have a longer stint.
These tyres exist in many real life racing series.
When it rains you won't bet only on when it rains but also how long. In changing weather conditions it should work.
In a totally wet race it could work if you are forced (by tyre wear) to stop 2 or 3 times in order to have a wet "dry" race with less tyre choice.
Maybe there will be too many tyres but this could help to not have boring wet races.