What would be an appropriate schema for this real life scenario?
I think this is a case where a super type and subtype should be considered.

The fields that are specific to an organisation go in a separate table to the main organisation, and if there are any feels that are specific to say a sub-org they are stored in a separate table and joined for crud operations.

This approach means any validation for say addresses applies to all organisations, and the validations (access permissions) for sub types apply to all entities in the relevant table.

This approach also eliminated excessive quantities of null fields.

Another possibility is the parent-child model using the same table. This is where the table records a parent which refers back to an I'd of the same table. Only reason I suggest this as a possible option is as far as I'm aware the easiest way to enable unlimited sub-organisation levels. Querying this model is relatively complicated.