Based on your situation, it seems like you need to set up custom dimensions in Google Analytics 4 (GA4) for the different types of memberships you offer. Instead of lumping all memberships into one group, custom dimensions will allow you to categorize and separate the data for the main membership and the additional membership.
To set this up, you should navigate to your GA4 property and go to ‘Configure > Custom Definitions > Create Custom Dimensions’. From there, you should name the dimension appropriately (for example, ‘Membership Type’) and provide a description. Once you’ve created the dimension, the real task begins: mapping this dimension to a parameter in your data. You’d need to ensure that each membership type sends the appropriate data parameter upon purchase (or other engagement) that corresponds to the custom dimension you’ve created. This might involve working with your developers if this requires changes in how data is sent to GA4.
In the end, this adjustment would allow each membership type to be tracked separately, giving you a clearer picture of user behaviors across different membership options. By separating your data in this way, you’ll be able to run more accurate analyses and make more informed decisions.