Onboarding Friction for PSPs: Any Success Stories with API-First Solutions?

We're currently re-evaluating our PSP relationships, particularly on the acquiring side, and the onboarding process has consistently been a bottleneck. The endless back-and-forth for KYB documentation, often requiring custom development just to integrate their bespoke API endpoints, really adds to the operational overhead. It makes scaling new payment methods or diversifying our acquiring partners a much heavier lift than it needs to be.

Has anyone had genuinely positive experiences with a PSP that offers a truly API-first approach, where the onboarding from a technical perspective was smooth and the documentation robust enough to minimize the usual integration headaches? I'm less concerned with the specific fee structure right now and more focused on the time to market for new integrations and reducing development resources spent on vendor-specific nuances.