Are current Oyster fares from central London to Harold Wood and
Brentwood identical (a "mid-Essex" zone) and if so what would Oyster
charge for a journey from Harold Wood to Brentwood?

Poor example as Harold Wood is within Greater London. The single fare finder gives results that put Brentwood "beyond" zone 9.

The problem is the allocation of special zones for NR destinations beyond zone 6. That eats up the additional zones that Oyster can cope with. If TfL pricing could be applied to the extensions then there would be three existing zones that could be tacked on to each line out of London.