To a great extent can be replaced by a variety of synonyms that offer a slightly different nuance and tone in writing. Consider using words such as largely, significantly, extensively, greatly, substantially, predominantly, overwhelmingly, or considerably. Each of these words conveys an element of magnification or importance to the extent being described. Alternatively, you could opt for more colloquial options such as big time, all-out, or fully to emphasize the significance of what's being described. Context is important in selecting the synonym that best suits the tone and flow of the writing, but these options provide flexibility and variety to express a range of ideas and intensities.