Ideally you can synergies into 3 broad buckets:
(1) Revenue Synergies: comes from cross-selling,
(2) Cost Synergies: You have fairly covered this well. It’s hard synergies I.e. reducing duplicated functions, procurement synergies, etc.
(3) Financial Synergies: They could be tax synergies (profit making company buying loss making company, thereby reducing taxes), capex synergies, lower cost of debt for smaller company, etc.