Insert of records into SQL Server is slow whereas select is quicker

Question

I am trying to improve the performance of insert in SQL Server.

INSERT INTO [dbo].[GroupCustomerAccountProductRebates]([GroupId],[GroupName],[ProductId],[ManufacturerProductCode],[ProductDescription],[CustomerAccountId],[CustomerAccountNo],[CustomerAccountName],[RebatePercentage],[DatePeriodId])
    SELECT GroupId, GroupName, ProductId, ManufacturerProductCode, ProductDescription, CustomerAccountId, CustomerAccountNo, CustomerAccountName, SUM(CAST(RebatePercentage AS decimal(18,2))) AS RebatePercentage, @DatePeriod
    FROM
        (SELECT * 
         FROM [dbo].[fnGetCustomerAccountFlatRateProductBrandRebatesByProduct] (NULL)

         UNION

         SELECT * 
         FROM [dbo].[fnGetCustomerAccountGroupInheritedFlatRateProductBrandRebatesByProduct] (NULL)
        
         UNION
        
         SELECT * 
         FROM [dbo].[fnGetCustomerAccountFlatRateProductRebatesByProduct] (NULL)

         UNION 
        
         SELECT * 
         FROM [dbo].[fnGetCustomerAccountGroupInheritedFlatRateProductRebatesByProduct] (NULL)

         UNION 

         SELECT * 
         FROM [dbo].[fnGetCustomerAccountTieredRebatesByProduct2] (NULL,@DatePeriod)

         UNION
        
         SELECT * 
         FROM [dbo].[fnGetCustomerAccountGroupInheritedTieredRebatesByProduct2] (NULL,@DatePeriod)
    ) x
    GROUP BY 
        GroupId, GroupName, ProductId, ManufacturerProductCode, ProductDescription, CustomerAccountId, CustomerAccountNo, CustomerAccountName
    ORDER BY 
        CustomerAccountName, ProductDescription

This is my query.

The SELECT query takes around 36 seconds for those 820,000 rows, but running with insert query takes 3 minutes for the execution.

Below link is the execution plan : https://www.brentozar.com/pastetheplan/?id=B1zcWas8c