Cloudflare servers need to be the only nameservers in your whois. You can use a partial setup to cover certain scenarios if you are on a business or enterprise subscription, but that still won’t involve mixing Cloudflare with other nameservers at your registrar.
There are a few setups with Cloudflare nameservers that will be of interest. The most likely candidate is with Cloudflare as Primary with outgoing zone transfers to the secondary. All of the “advanced” setups are available to Enterprise customers only. Some details here:
You are right, for an undetermined amount of money Cloudflare will do zone transfers, and undoubtedly other special services.
But what I was most surprised at and commenting on was their unwillingness to be one of a number of independent providers listed at a registrar until you are at that pricing tier. (not even planning on AXFR, just independent updates.)
Enterprise pricing seems opaque, and undoubtedly overkill for the strictly DNS-only redundancy service that we are looking for.