I'd love to learn the secret incantation from "senior enough" employees who have managed to negotiate these kinds of things. I've been at it for 25+ years and only managed to negotiate pay (never large increases, and not from many companies). When it comes to your terms of employment like hours, vacation, benefits, sick time, and so on, it's always "The boilerplate employment agreement is written by god and cannot be changed by mortals. Sign it unmodified or GTFO!"