ECS Cluster stuck in a busy state for days after capacity provider changes

My ECS cluster is stuck in “The specified cluster is in a busy state. Cluster attachments must be in UPDATE_COMPLETE or UPDATE_FAILED state before they can be updated. Wait and try again.” for 2 days.

I tried to make some changes in the capacity provider, which seems to have brought the entire cluster into a stuck “updating” state. I've read a few answers here that indicate that this would fix itself within a few minutes or hours, but nothing seems to be happening.

When I run aws ecs describe-clusters --clusters <redacted_cluster_arn> --include ATTACHMENTS --query clusters\[\].attachmentsStatus I get: [ "UPDATE_IN_PROGRESS" ]

This cluster has no services or tasks. I tried removing everything – registered instances, ASG, services, to no avail. At this point, I'd just like to delete the cluster but can't. Can someone point me in the right direction to erase these blocked resources from my organisation?

Update: here is the cluster description

{
    "clusters": [
        {
            "clusterArn": [REDACTED],
            "clusterName": "ecs-cluster-name",
            "status": "ACTIVE",
            "registeredContainerInstancesCount": 0,
            "runningTasksCount": 0,
            "pendingTasksCount": 0,
            "activeServicesCount": 0,
            "statistics": [],
            "tags": [],
            "settings": [],
            "capacityProviders": [
                "cp-external-ecs"
            ],
            "defaultCapacityProviderStrategy": [],
            "attachments": [
                {
                    "id": [REDACTED],
                    "type": "as_policy",
                    "status": "CREATED",
                    "details": [
                        {
                            "name": "capacityProviderName",
                            "value": "cp-external-ecs"
                        },
                        {
                            "name": "scalingPolicyName",
                            "value": "ECSManagedAutoScalingPolicy-[REDACTED]"
                        }
                    ]
                }
            ],
            "attachmentsStatus": "UPDATE_IN_PROGRESS"
        }
    ],
    "failures": []
}