Could my payments be failing because of sandbox vs live API keys?

Yes, this is one of the most common payment bugs after a go-live or a server migration. If the live store still points at test keys you’ll see errors like No such payment_intent or This API key is a test key; if test environments hold live keys you can take real money in QA. We audit the per-website / per-store-view config, move secrets out of the database into env.php so each environment is correct, and verify a real transaction against the live endpoint.