Stuck with DNS questions during migration to AWS EC2 + Route 53

Stuck with DNS questions during migration to AWS EC2 + Route 53

We're running a little hosting service with two nameservers registered for
us at a local registrar:
ns1.ourcompany.com
ns2.ourcompany.com
Now i want to move over to cloudflare with all the DNS services. From
them, i got two new nameservers:
lady.ns.cloudflare.com
pete.ns.cloudflare.com
It is a killer requirement for us that we do not loose the existing two
nameservers because they are already configured in customers top-level
domains where we don't have access directly.
Now, i'm wondering what my options are:
Find out what ip's the two cloudflare servers really have and change the
entries for our existing nameservers to these ip's -> pro, solved; con,
the ip's can change and silently break my dns setup. More lookups are
necessary to finally lookup a customers top level domain.
Making an CNAME alias for my existing nameserver to point to cloudflare
servers directly, but this is against the RFC of DNS, no one will do that
for me..
Pay a business account for at least 200$ per month at cloudflare to get
custom named nameservers
Are there other DNS providers which has an API for automation and offers
custom/vanity nameserver for a payable price per month?