I found that the cause of no other calendars showing was needing to make sure our OAuth consent screen in Google added the scopes for the calendars. However, I have also found through testing that only the calendars that exist at the time you create your Google API project show up as an option (this is for self-hosted MachForm). If you create new calendars you would like to use, it appears you have to remove all integrations from MachForm, revoke access to your Google integration account for the MachForm product, remove and recreate a new Google API project, and then re-create/setup the integration between MachForm and Google.