The operational problem
A business leaving a wider group may still depend on a shared Salesforce org. Customer records, applications, automation and integrations have to be separated without carrying across unrelated group data or years of unnecessary configuration.
The target also depends on work outside Salesforce: system owners may need to change endpoints, credentials and their side of each connection.
Move the agreed scope
Start with the processes the independent business needs. Identify the relevant configuration and packages, map the in-scope data and its relationships, then plan how each external service will reconnect.
Use a newly provisioned org to establish the target, rehearse the migration and test the business journeys. Keep new functionality separate from the carve-out unless it is required for independent operation.
Plan the operating transition
User acceptance testing, training, the final load and a defined period of post-release support belong in the same transition plan. Assign ownership for every dependency and exception.
Acceptance should establish that the agreed processes work independently, the required records and relationships are usable, and the external connections behave as expected.