I’ve never used Scribus, but it does seem a useful and free WYSIWYG Layout app.
Pandoc doesn’t ssupport Scribus natively, but someone has written a custom writer, a plugin that allows Pandoc to output a Scribus file:
It converts almost all block and inline elements to Scribus styles and can use a Pandoc template if you want to allow custom metadata to drive content. This should mean a style-based Scrivener to Pandoc workflow should translate to Scribus without too much work.