Hi, external DNS is the automated way of updating or creating route53 entries. I am not aware of any other techniques. However, when you start moving to a new cluster, it is a must for cluster to create a new load balancer for each of your services (depending on configuration) therefore external DNS will handle updated in an automated way.
To be fair with you, once you have your new k8s cluster ready and networking configured for different RDS etc. using VPC peering or transit gateway, updating route53 using external DNS will be the easiest and most pleasant part of your migration.