The reason you get staves overlapping is that Dorico has to estimate how much music it can fit on the page while it’s casting off. It doesn’t know at that point exactly where everything will end up, and it thinks it will be able to fit two systems comfortably onto pages 3, 4, and 5, but the fact that there are dynamics above all staves on all systems prevents this from being true. I agree with Frank’s advice that reducing the space size on the Page Setup page of Layout Options is probably the best remedy.