Apologies if this has been posted before, or implemented without me finding how to do it.

Would be great if there'd be a button for jumping to the next top-level comment. Possibly at the lower-right corner, as some other apps. Long-press for previous would be great, too.

Yes, I know there's support for collapsing comments. But you have to have it in view to do that. If you're browsing a long thread and want to jump to the next top-level comment, it becomes a bit of a chore.