6-3-5 brainwriting is a structured, silent ideation method where 6 participants each generate 3 ideas in 5 minutes across 6 rounds, yielding up to 108 ideas and enabling equal participation, rapid ideation and cumulative development. Many ideas are fundamentally visual, making generative image models especially useful for accelerating visual exploration and making abstract ideas quickly tangible, but existing tools cannot fully support visual composition, where ideas are built from atomic elements and iteratively remixed. A formative study (N=18) showed that even with GenAI embedded in a 6-3-5 structure, participants invested effort in crafting monolithic prompts yet struggled to obtain intended results, build on others’ ideas, or work modularly, leading to redundancy and limited divergence. To address these issues, we present AtoMix, a collaborative visual interface for remote 6-3-5 brainwriting that supports granularity and cross-pollination through atomic prompting, canvas composition, and reuse of image fragments. In a comparative study (N=24), AtoMix afforded more fine-grained interaction than the baseline and better supported collaborative visual ideation.