Wow, so you did mean that. No, that’s not even a remote possibility. The NS records for the domain in question would have to point at DO before their nameservers would respond to requests for your domain. There’s no means to hijack a domain in this way.
In the case of Cloudflare it’s an 8 millisecond request every five minutes, in the case of DO it’s ~14 milliseconds every 60 minutes. I’m impressed if you’re saying you can tell the difference.
None of the above is a case to move from the likes of registrars such as GoDaddy or Namecheap over to DO. From what you’ve admitted the decision appears to have been led by a fundamental misunderstanding of DNS.
In the above comparisons we didn’t touch on the other differentiators that CloudFlare offers, features such as inbound rules, apps and their reverse proxy.
I do hope you revisit this because you’re doing your customers a great disservice even suggesting migrating DNS to DO. As you’ve already mentioned there’s the risk that they may move a domain without preserving records (unlike cloudflare they don’t appear to grab DNS records prior to transfer), there’s zero meaningful performance benefit, and a much greater chance of downtime during any migration.