From what im reading here and i could be wrong, once you disassociate an AWS Account from an existing aws organisation. the AWS Identity Centre will not work.
If you have enabled trusted access for AWS IAM Identity Center (successor to AWS Single Sign-On), you can pick multiple AWS accounts whose users need single sign-on (SSO) access AWS accounts and cloud applications. When an account leaves the organization, IAM Identity Center will no longer, enumerate the account, or be able to deploy common sets of permissions, and manage access from the management or a delegated administrator account. IAM Identity Center automatically cleans up the account of any metadata and resources, including the IAM Identity Center service-linked role. The account no longer works with IAM Identity Center and you can no longer access the account using the AWS Access Portal for any configured identity source.
When an account joins the target organization, you will need to configure required permissions or assignments to users and groups in IAM Identity Center for the account.```