Morning,
I can help a little bit here. You won’t be able to use Avanti trains as although they call at Watford Junction it’s a pick up point on journeys from London and set down only on journeys to London.
You can use Oyster or contactless on the Overground. There are Oyster readers at both Euston and Watford Junction.
You will need to purchase a ticket to use London North Western trains. They don’t accept Oyster or contactless.
I’m not sure of pricing but I imagine Overground should be the cheapest and you get the benefit of capping.
Chris