Thank you for providing the details.
I can surely help you with removing the CBC cluster and cloud from the Couchbase Cloud UI. However, since the CF stack has already been deleted in your AWS account which in-turn would hinder our control plane from performing necessary actions in your AWS account, we can not confirm that all of the infrastructure will be deleted after removing the cloud from CBC UI as we do not have access to your AWS account to delete the resources anymore.
This is the reason that we always recommend to follow our step by step guidelines in order to delete the cloud/cluster from the CBC UI which ensures clean and complete deletion of resources and helps in avoiding any such issues.
As you have already mentioned that KMS has been left behind, there could be other resources that are not VPC based that might also be left such as EBS, EKS clusters etc. Hence, I request you to please ensure to delete those resources manually to avoid any cost incurred.
Also, please DM your email ID associated with Couchbase Cloud account so that we could reach out to you in order to help further.