> so a restrictive DMARC policy (p=reject; adkim=s)

This isn't how DMARC works. Valid SPF OR DKIM will validate a message. So unless you drop your SPF policy DMARC will still pass. And unfortunately if you don't have a good SPF policy many mail servers will send your messages to the spam folder, so there isn't a good way to have only DKIM set up.

There are some discussions about requiring DKIM independent of SPF, but nothing published (nevermind adopted) yet.