"Wax and wane" is a popular phrase that refers to the constant rise and fall in the intensity of something, such as the tides, moods, or fortunes. There are several synonyms that can be used in place of "wax and wane" to describe this phenomenon. Some of these synonyms include "ebb and flow," "rise and fall," "fluctuate," "cycle," "oscillate," "alternate," "vacillate," and "undulate." All of these words convey the idea of something that changes in a regular or irregular pattern, often in a cyclical manner. So, if you want to switch up your vocabulary and sound more sophisticated, try using one of these synonyms instead of "wax and wane".