"Mull over" is a commonly used phrase meaning to think about something carefully and thoroughly. However, there are several synonyms for this phrase that can be used to spice up your writing and add more variety to your vocabulary. For example, you can consider "ponder," which means to think deeply or consider carefully. Another synonym is "ruminate," which means to meditate on or reflect upon something deeply. "Contemplate" is another word that can be used as it means to consider thoroughly and deeply. Finally, "deliberate" is another synonym that means to carefully weigh and consider something before making a decision. Using these synonyms instead of "mull over" can help your writing sound more compelling.