Ortleib rack mounts can handle more shaking than any rider can. They should have come with some small plastic shims for narrower rack sections. make sure that the anti-sway hook is positioned to engage the rack struts tightly.
I have ridden steep, rough, rutted, boulder-strewn mountain trails, fully loaded on 32mm tyres and it is very tricky. I often wished that I had 38mm which seems to be the modern standard for touring bikes.