How would you fix these rotten window sills? It's 2x6or8 under very large windows i'm not taking out. Had rotten trim that matches the sides. I'm not sure about cutting half of it out right next to glass (which im not sure can be replaced). Would it be dumb to use a ton of wood filler and flashing?

It's a cheap kit house from the 70's, so i'm thinking about cheap but good fixes that kind of match.