The difference between your Chelmsford to London ticket and the Chelmsford to London + zone 1-6 is £1,040. That's a lot of money.
The daily cap for zone 1-2 is £6.80. Zone 1-3 is £8. If you were using the tube etc to travel in zone 1-3 more than every 2.8 days (so more than twice a week), it might make sense to get the zone 1-6 travel card. If not (and I'd suspect not), you'll save money by using pay as you go.
If you were ever spending a week in London, you could get a weekly travel card. A zone 1-3 weekly card is £40, so basically 7 days for the price of 5. Zone 1-2 is £34.10, so the same deal.
Apologies if any of my math is wrong.