Forum discussions in Massive Open Online Courses (MOOCs) play a crucial role in promoting learning engagement and academic achievement. In particular, discussion topics significantly influence learners' emotional and cognitive engagement. However, the complex interrelationships among these factors remain underexplored. This study introduces an innovative two-step methodological approach to investigate the relationships between topic complexity, emotional engagement, cognitive engagement, and academic achievement in MOOC discussions. Using BERT for engagement detection and developing a Joint Emotion and Cognition Topic Model (JECTM) based on Bayesian networks, we analyzed 27,428 discussion posts from 2857 learners in a psychology MOOC. Our findings reveal three key insights: (1) The proposed two-step approach efficiently detects topics and analyzes their patterns of emotional and cognitive engagement. (2) As topic complexity increases, learners demonstrate higher-order cognitive engagement while experiencing reduced positive emotions along with increased confused and negative emotions. (3) In high-complexity topics, learners who maintain both positive emotions and higher-order cognitive engagement are more likely to achieve academic success than those who have negative emotions or lower-order cognitive engagement. These fine-grained analyses provide valuable insights for optimizing discussion design and interventions. This study also provides implications for the analysis of classroom dialogues and AI tutor-based conversations.