I always carry a poncho, even in july/august.
Usually it is needed for only a few days during those months, but without it, those days would be pretty miserable (when you're used to 30-40C heat waves, a sudden temperature drop to 15 or even 20C rain and wind, for example during a lightning storm, can feel freezing cold). An umbrella might keep you more or less dry, but it doesn't keep the wind off. A rain jacket or poncho adds warmth, too. A poncho also doubles as a picnic blanket.
I'd never hike in a mountainous area without adequate rain gear. The weather can still change quickly, even if it's "only" at 1200m altitude or so. If you get hit by bad weather in the pyrenees for example, with only an umbrella, that's a good recipe for hypothermia even in summer.
There are a few essentials for long distance hikes, and rain gear is one of them, in my opinion, no matter what season.
150-300g for a lightweight poncho, that's the weight of an apple. If weight is a concern, there's surely something else than can be left at home or be replaced with something lighter
Buen Camino and happy planning!