Hi Juergen,
The posting setup can be made in a way that a specific ledger account is defined at the item level. You do thus not require hundreds of item groups but can continue with the ones that you have. What is different is that you have to define a separate ledger account for each of your service items. Probably test this in a demo environment to verify the differences to your current process and the implications that those changes might bring.
Best regards,
Ludwig