PnP JS Search API - Get/filter all user created docs in a site collection

Question

I would like to get all user created documents in the whole site collection using PnpJS Search.

const results = await sp.search({
  Querytext: `
     contentclass: "STS_ListItem_DocumentLibrary",
     IsDocument:"true",
     path: "http://mysp.net/sites/mysite/*
   });
  `

})

Unfortunately this still shows /Site Assets/ docs which are not user created. Obviously I can filter out these after the search - but wondering if it's possible to do an exclusion in the search? Is there a better way to do get all the user created docs or filter out the system generated ones - maybe using RefinementFilters or some other contentclass ??

This is sp2013