Use a tool like Miro or a physical board. Use a different color for each research activity, e.g. usability studies = blue.
You will end up with a matrix of key takeaways color-coded by research activity.
Group the key takeaways into themes. Look for duplication or similar topics that can be grouped into larger themes.
The insights don’t have to be exactly the same. but should relate to a larger topic, e.g. navigation, content, pricing, delivery, etc.
For example, Pricing, Navigation, or Value proposition.