The problem is, history DOES sync, but tabs don't. There was a period where a stale list of tabs did show up, both on my iPhone and on my MacBook, but that didn't update. After toggling the safari thing in the iCloud settings on iOS and macOS, the tab list stopped showing up at all. Now safari is enabled in the settings and the history syncs, but open tabs don't!
Any pointers on how to debug iCloud? How does it actually sync safari stuff?
 Forgot to mention, bookmarks sync too. It's just tabs that don't.