I think the first thing I'd do with something like this is to put your plans into the
TfL single fare finder to work out how much it would cost to do everything you intend to do with a contactless card. This may be affected by capping, depending on how many other journeys you intend to make, but I don't know how that works.
If that's not an option, and if you're not going to Watford until after 9:30, one possibility is:
£13.50 anytime travelcard zone 1-4
£11.10 anytime day return Maidenhead-Southall
£5.90 off peak day return Boundary Zone 4-Watford Junction
£30.50 total
You don't need to catch a train which stops at Southall in either direction. It's simply a station in zone 4 which is on the line I assume you would use to get into London.