@Ernest Aduboffour Thanks for reaching out. As per this https://github.com/MicrosoftDocs/azure-docs/issues/94436 you will not be able to grant access to purge the APIM instance at resource group level.
To purge a soft-deleted instance, you must have the following RBAC permissions at the subscription scope in addition to Contributor access to the API Management instance: Microsoft.ApiManagement/locations/deletedservices/delete, Microsoft.ApiManagement/deletedservices/read. please refer: https://learn.microsoft.com/en-us/azure/api-management/soft-delete#purge-a-soft-deleted-instance
do let me know incase of further queries, I would be happy to assist you.