I lived in essex for 25 years, school and worked all over really. I lived in a village called hatfield peverel along the A12 corridor. Quite a nice place and on the edge of some good countryside and riding.
Some pretty good advice already – as a rough rule I would say draw a line north south through chelmsford and then try and head as far east and north that your journey times will allow. With the exception of witham, the further east you go the better essex gets. Beautiful estuaries and coastline, pretty remote too. Off road is suprisingly good although not technical, road biking good too. Perfect xc and cx terrain
I’d think about maldon and the blackwater estuary area, but maybe on the limits of travel times as Essex is a busy place and I think youll find Southend is 45 mins from chelmsford, if not more. Personally I wouldn’t contemplate south of the A127. It’s truly wretched.