The best way to reduce a 3D model with texture atlases balances texture downscaling, mesh simplification, and critical detail preservation for optimal size and quality.
Lower texture resolution (e.g., from 4K to 2K) to cut file size without losing essential clarity. Use mesh decimation tools to reduce polygon count, focusing on retaining key features like character facial details or product edges.
For tools like Blender or Substance Painter, start with minor texture adjustments and test level-of-detail (LOD) settings in your target use case (e.g., game engines) to avoid over-reducing.