The integration of blockchain smart contracts in Business Process Management (BPM) enhances trust and transparency in cross-organizational process execution. However, traditional smart contract-based solutions face scalability challenges when handling large or complex business process models in Solidity-compatible blockchains. Additionally, existing architectures lack flexibility, requiring full redeployment and state migration for updates to model specifications or execution engines. To address these issues, we propose a novel smart contract architecture based on the diamond software pattern. This architecture enables the execution of large-scale BPM models, as it maintains a constant contract size regardless of the model’s complexity. The modularized execution logic also improves upgradeability by allowing updates to specific components without redeploying the entire system. We evaluate our approach using a variety of process models. Our experiments demonstrate that the diamond-based architecture can successfully handle models with up to 100 elements, while state-of-the-art solutions struggle with models exceeding 50 elements. Furthermore, the gas costs associated with deploying the diamond infrastructure are mitigated after running three instances, making the architecture particularly suitable for multi-tenant collaborations, where participants can share the same execution engine across models and instances.