The proper timetables will be published 12 weeks before travel, and I would use the National Rail site to figure out the timings, not TfL.

If you're in West London, bear in mind there is an hourly, direct service to Watford Junction (it's the South Croydon to Milton Keynes train) from West Brompton, Kensington Olympia and Shepherd's Bush. Depending on your location, that can be very convenient.

IIRC Watford Met is not long for this world, as the Croxley branch is being reinstated.