Love for Mollie - and literally had this exact theme last year at work. Stripe implemented, then customer A couldn't use it due to US base, so went to Adyen, built integration, rejected as less than $5 million as first responder said, then went to Mollie.

Only gripe is no embeddable checkout but its not a huge deal, and they have superior test platform than even Stripe. The test cards are right there in slide in panel, and you have option to select paid/cancel/fail etc to test different outcomes.