Unfortunately, you will not be able to get the permissions at higher level - you will have to go through each item and get a unique permission at the item level and then gather the list.
You mentioned that , you may not be able to get the full list - I could think of edge that will cause this.
/sites/{siteId}/drive/items/{itemId}/permissions
Alternatively, if you like to know all the users - at the site level - you can query the User information list using the Sharepoint ReST API.
_api/Web/lists/getbytitle('User Information List')/getitems
You could filter for all active users - this will not provide the permisssions possesed by them - but gives a list of users added to the site.
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