How do I force a page break before a section?

Select the section’s container, and in the properties panel set its page-break to break before: always. That block will then start at the top of a new page every time, and it’ll move with content above it if things reflow.

Tip: put it on the section wrapper, not on the heading inside it, that way the whole section travels together. There’s also “avoid break inside” if you want to stop a single block (like a chart + caption) from being split across pages.