Yes I have done the unthinkable of mixing my internal domain name with externally resolvable, such as this blog for instance: blog.rylander.io.
So I want my blog.rylander.io address to be available both from within my own network as well as on the Internet. I do run a Windows Domain and a DNS server.
On the domain, right click and select “New Delegation”. When asked, enter the name of your sub-domain.
Add the NS servers given to you by Cloudflare. In my case desi and phil.
A new leaf is created and subsequently all queries for the sub-domain will be delegated to the specified servers.