Ok, so in my opinion, rather than delegating the subdomain’s DNS to Digital Ocean (DO) I would simply add them in the Custom DNS section of the admin area. This will put the DNS all in the same place.
As I said earlier, you could delegate it, but it is complex. IF you want to delegate the subdomain’s to DO you need to create each subdomain as a domain in DO networking to properly work.
Please read this to understand what that area of the Custom DNS page is: