In the article " How to configure DNS for CNAME (partial setup) when managing DNS externally"
It say:
"This KB article is designed for partners who want to add domains on CNAME setup and use the Cloudflare Partner (Host) API while managing their authoritative DNS externally. "
Not sure why your host needs to retain control of your DNS for email, but assuming that is true you could sign up in a CNAME setup with a Business plan.