Seeking for advices: AI writing tool with custom (and large) dataset

Hello, I'm looking for advice on choosing an AI / AI tool that I can customize to help me write articles. My issue is that I work in a very specific niche and need this AI to pull from a database of around 8,000 pages. This is necessary to tailor my responses and source my content effectively. The paid versions of ChatGPT and Gemini don't accept this volume of data. Any recommendations? Ideally, this AI should also have SEO data so that the semantics are optimized.