Hi,
As Feathers says, the one day travelcard doesn’t include Shenfield, anything west of West Drayton, and if it’s only zones 1-6 then it also doesn’t cover Moor Park to Amersham. If you don’t have a railcard then I think using contactless is going to be cheapest as you only seem to be going one way from Reading. It should charge you a zone 1-9 cap plus a single extension fare from Reading to Boundary zone 6 and two singles between Brentwood and Shenfield. If you do do this then I’d be very interested to see your journey history to check how it has worked it all out.
If you do have a railcard then I think the following tickets will work out chaepest:
Amersham to zones 1-6 day travelcard (aaa zones 7-9)
Return from Brentwood to Shenfield
Single from Reading to West Drayton
Hope this helps