Domain Registrar Management for clients?

Is there a way of giving ownership of the domain to a client after registering it? or opening another CF account for the client to own the domain whilst we manage it?

It sounds like you need to use Multi User accounts:

cool, so for the domain to have the clients details attached, the superadmin would be the client (billing etc.) and the developer would be added as a member with admin role? (free plan)

It’s up to you which way you flip the admin roles. Some agencies will create a new super admin account for every client, then add their own main account as a regular admin so you can admin every account centrally. You may or may not want to share the super admin account login details with your client. You can also add a third regular admin account if you don’t share those super admin credentials, so the client can log in as well.

EDIT: Now that I think about it, I believe you’ll want to create a super admin account for each client, as that’s the “owner” of the account. I don’t think you can make your current account a super admin for another account.

and the “owner” details of the super admin account would be used as the “owner” of the domain registration?

Absolutely. There’s no way around that, as that’s who controls billing.