How to manage ECS Clusters across accounts?

First of all, I don't think there is a tool quite like Lens, for ECS. That is, centrally manage and store credentials of various K8s clusters and provide single plane of glass for cluster management.

That said, it seems that your main concern is don't want to logging back and forth between accounts. If that is the case, there might be couple of options.

1. Use tools like CoPilot, and add AWS CLI named profiles to store credentials for different accounts, and use them when needed;
2. Use cross-account IAM roles to grant specific permissions to an user who need to have access to multiple ECS clusters across accounts;
3. Utilize centralized CI/CD process where the Pipeline is in one account, deploying applications into multiple accounts using cross-account IAM roles.