Default to root-relative URLs in links

You mean you want to remove the domain from the permalinks? To do this you would have to change the permalink generation. Here is an answer that describes how to do this and what problems there might be:
https://stackoverflow.com/questions/3472334/is-there-a-way-to-exclude-domain-from-link-generation-in-wordpress

The biggest problem from my point of view: you are not allowed to customize all permalink generations here, e.g. not those for the SitemapXML and also not those for the canonical links. The specification of the domain by WordPress makes sense.