Formal:
* Principally: This is a direct synonym of "primarily."
* Chiefly: Emphasizes the main importance.
* Mainly: Similar to "chiefly," but slightly less formal.
* Predominantly: Suggests a clear majority or dominance.
* Essentially: Focuses on the core or fundamental aspect.
Informal:
* Mostly: A common and simple alternative.
* For the most part: Indicates a general trend.
* In the main: More formal, but less common than "mainly."
* Above all: Highlights the greatest importance.
* First and foremost: Emphasizes the highest priority.
Other options:
* Fundamentally: Focuses on the underlying cause or reason.
* By and large: Indicates a general truth with some exceptions.
* Generally: Indicates a common occurrence or trend.
The best word to use will depend on the specific nuance you want to convey.