Not only that is a commonly used phrase that indicates the continuation of a thought or idea. However, there are several synonyms that can be used in place of this phrase to add variety to your writing. Some alternatives include: furthermore, in addition, additionally, moreover, what's more, besides, and also. These words can help to emphasize the importance of the information being shared, adding depth to your writing. When used correctly, synonyms for not only that can also help to create a natural flow in your writing and engage your readers.