The synthesis is not just a summary of each reading that you've decided to include in your review. The purpose of your synthesis is to bring together all of your research findings to:
You're going to need to sort and collate your references by main topics and themes, so you can see which arguments they support. This allows you to pull these ideas together to frame coherent arguments and provide supporting evidence from what you've read.
This is where your literature matrix is useful. You can sort it by category (heading). For example, you could sort it to group your references by key themes or by the section of your research question they are related to, which makes it easier to start your analysis and synthesis of your findings.
If your original matrix is too unwieldy for this stage, you could use copies of it create smaller, separate matrices for each theme or topic or section of your review. The original matrix will retain all of your information, but the smaller ones only need the columns of information that are relevant to this process. This means that you have less information to wade through and enables you to focus on each theme or section one at a time.
Note: There is a large volume of work in this stage of your review so breaking it up in to easier stages is a good idea.