Our initial Oyster cards were for 30 and we had to refill with 10 on our last full day. So we used 40ish over 5 days. You are capped a certain amount no matter how much you ride in a day. Every person needs their own card.
The other option is to use a contactless credit card. Make sure the card provides free exchange rates. Again, every person needs their own card.
Apple Pay is available at many, many places. I used it for a pay toilet once. Wow.
If you pay at a restaurant with a card, they will run it at the table on a wireless device. If the serivice charge (tip) is not listed on the check, then tip 10%.