One thing we used to do in running was to wear two very thin and light water resistant tops together.
Didn’t get the boil in the bag, its a cheap option, pretty light and near enough fully waterproof (rain gets through after about 1.5-2 hours)
You can wear one layer normally to keep wind chill off and then put on the second layer when its starts to rain heavier.
My current favourite set up, although its not cheap, is to use a Gore ALP-X soft shell set up for normal riding and add a lightweight water resistant top over that when the rain gets heavy. I accept this does not truly answer your original, moon on a stick, question