As Richard implies, almost everyone taking the Haruka will be in your position, i.e. buying a walk-up ticket.

Your arrival falls in the middle of Golden Week, so most residents of Japan will be in the middle of their vacations, not coming or going. If you can't make your first-choice train, you can most likely make the next one. JR may even add extra cars to the trains if the standard configuration sells out.

Your departure is at the end of Golden Week but on a Saturday. The big rush will be on Sunday, but if you are worried, you can reserve your return ticket when you buy your ticket to Kyoto.