Traveling with an Amex - will I be able to use it widely in Sao Tome and Principeor should I carry a backup Visa/Mastercard?
No, you won't be able to use Amex widely. Your best bet is to get cash from an ATM with a Visa or Mastercard. While a few places might take credit cards, they are not common, and even the ones that do might charge you an extra fee. It's much simpler to just use cash and have a backup Visa or Mastercard for the rare occasion a place takes it.
I'd strongly advise against relying on your Amex. I was there a couple of months ago, and my Amex was a total bust. I saw maybe one or two places in the capital that would even consider it. The infrastructure just isn't there. Your best bet is to bring a Visa or Mastercard for the few places that accept cards, and otherwise, plan to use cash. You can exchange Euros or US dollars at a bank or hotel.
What a great place! But you are absolutely going to need a backup. In my experience, even places that have a credit card machine will have trouble with Amex. The local banking system, with banks like Ecobank and Afriland First Bank, is heavily reliant on Visa and Mastercard. It’s also important to know that many places, especially outside the capital, are cash-only. So, bring a Visa or Mastercard and lots of cash.
You definitely need a backup card here. I was just there last year and found that Amex is not widely accepted at all. The only places that will reliably take it are the high-end, tourist-focused resorts, like the HBD Príncipe hotels. For all other businesses, including restaurants, guesthouses, and smaller shops, you'll need either cash or a Visa or Mastercard. In fact, most ATMs in São Tomé and Príncipe only work with local bank cards, so you'll also want to bring a good amount of Euros or US dollars to exchange.