My experience with "the lightest rain gear you can buy" was disastrous.
Before my first Camino, I looked into ponchos and rain jackets extensively, and thought they were awfully heavy. I then discovered super light weight ponchos, the kind you carry in your glove compartment for emergencies. They were so light, about 1.3oz, and cheap, that I bought a dozen and gave three to each of my two sons, my wife and myself.
They worked great - until it rained. Then, one day, in Galicia, in an area nowhere near any town and without any cell coverage, we were hit with heavy rain, driven nearly horizontal by roughly 30 knot winds. The ponchos lasted, maybe, 15 seconds before they were torn to shreds. Having three each did not help; 3 x worthless = worthless. We were soaking wet and in serious danger of hypothermia. I don't know how the story would have ended had a local family not taken us in, let us dry off, fed us, given us coffee, and driven us to a bar about 10 kilometers away which rented us a room.
You may not get rained on while walking the Camino. If you do, it might be a pleasant change on a hot day. On the other hand, it might be cold, no matter when you walk, and, without rain gear, you may be in serious trouble. Hypothermia can and does kill. My advice is to be prepared.
We now carry Packas, a sort of combination poncho and rain jacket which can be mounted on our packs like pack covers, and put on like a jacket in a moment if it rains. Mine weighs 11.5 ounces, and which is weight I am happy to carry.