To add collision to a 3D sprite model without a high poly count, use simplified primitives like boxes, spheres, or capsules that approximate the model’s shape instead of detailed meshes.
These low-poly shapes are computationally efficient—they don’t need to match the sprite’s polygon density—and most game engines (e.g., Unity, Unreal) let you assign them as collision components directly.
For slightly complex models, combine a few primitives (e.g., a capsule for a character’s torso + boxes for arms) to keep performance high while maintaining basic collision accuracy.