Localize English marketing copy into Castilian Spanish Open

I'm looking for a localization and copywriting service for one of our Spanish clients. We are Growth consultancy, and insist every idea is A/B tested on our client's websites to verify we've measurably grown their business. Copywriting (in English) is one of our core strengths, however one of our Spanish clients is not impressed with the translation services we've been using. So far, translations have been accurate but too literal, and sound clunky to a native Spanish . So we're looking for someone who can accurately capture and translate the core messages we're trying to test, while also ensuring the tone resonates with the client's audience. While our needs may vary, we'd typically be looking to translate a steady 250-500 words per month for as long as our engagement lasts with the client