I was in the same boat and obsessively checked the forecast even though I knew it could change. Even on the day that looked like it would be rainy and thunderstormy all day, it wasn't that bad and there were plenty dry spots. Good shoes, a rain coat, and cafe visits meant the day was still great for us. Getting caught in a hail storm (yes, hail!) at Sacre Coeur sounds bad from home, but when you're there, in Paris, it was just another memory made. I missed out on visiting Pere LeChaise due to weather, but that was the only thing we skipped that day.
Advice: don't stress about weather this far in advance. It will change. Check a few days before for packing needs and then trust that Paris in the rain is better than your normal day back home :)