Fenchurch Street is the C2C terminus from Southend, but the stations are all very close together in that area. Neither Redhill nor Merstham are actually in zone 6, they are outside the zones but accept Oyster because they are on the way to Gatwick.
From those stations Oyster isn’t usually cheapest. You can’t load season tickets onto Oyster from Redhill or Merstham, so you’re better off using paper or the Key smartcard.
At the moment the Northern line is shut through London Bridge so you should either change at Blackfriars if using a Thameslink train or take a Southeastern train to Cannon Street if your train from Redhill terminates at London Bridge. From Blackfriars or Cannon Street take the District line to Tower Hill.
Hope that helps a bit.