Which subdomain should I put the WordPress on for a headless site?

The convention is clear: your front end (Astro) lives on the root domain (yourdomain.com) and the WordPress on a subdomain like cms.yourdomain.com, which only you use to edit.

And the nameservers?

Here’s the usual mix-up: where your nameservers sit doesn’t tie down where your hosting lives. You can manage DNS wherever you like (your registrar, Cloudflare, Vercel) and all you need is an A record pointing cms at the IP of your WordPress host.

That way they coexist without a hitch: the front end on an edge platform, the CMS on PHP hosting and the email on a separate service, all under the same domain.