Help, I don’t understand how to travel in London

I feel so dumb I just landed at Heathrow for a 7-day trip and I think I messed up.

Not knowing the system, I bought an Oyster card with a 7-day zone 1–6 Travelcard (~£95). I tapped in at Heathrow and took the Heathrow Express to Paddington. When I tried to change at Paddington, the gate said my Oyster was empty and needed topping up, even though I’d just paid £95?

I then realized everybody used their debit or creditcard so I tried tapping in with my Visa credit card, which declined, and ended up using my debit card instead (which did work!)

My travel this week is mainly Tower Hill ↔ ExCeL London, plus some trips within the city.

Questions:
- Is my 7-day Travelcard really depleted after one journey, or did something else happen?

- Should I just abandon the Oyster and use contactless debit for everything? Can I get a refund?

- Was zone 1–6 overkill for my kind of travel?

Any help appreciated!!