The increasing complexity of modern software systems underscores the need for robust, automated modeling tools. Among Unified Modeling Language (UML) artifacts, Sequence Diagrams play a crucial role in depicting dynamic interactions between system components. However, their manual creation remains time-consuming and error-prone. This paper extends our framework, focusing specifically on the synthesis of Sequence Diagrams. We introduce a two-stage pipeline that combines a lightweight language model (LLaMA 3.2 1B-Instruct) for generating detailed technical specifications, with a reasoning-enhanced large language model (DeepSeek-R1-Distill-Qwen-32B) to produce corresponding PlantUML code. This approach yields a novel dataset of 1,000 high-quality samples, each containing a technical description, PlantUML code, and the resulting diagram. To validate semantic and structural fidelity, we …