Task-based research has often overlooked individual differences (IDs) and task-readiness factors in developing instructional materials and curricula. This study addresses these gaps by examining how L2 writing anxiety influences the Complexity, Accuracy, Lexis, and Fluency (CALF) of writing performance across tasks with varying cognitive demands under two task-readiness conditions: task repetition and task rehearsal. Ninety undergraduate ESL students completed a questionnaire on L2 writing anxiety before performing two argumentative tasks of differing cognitive complexity, administered one week apart in a counterbalanced design. After completing the first set of tasks, participants filled out a perception questionnaire to validate the task complexity manipulation. They then repeated the same tasks within the same timeframe. The findings revealed that while anxiety positively affected syntactic complexity, it negatively impacted accuracy overall. Under task repetition (implicit preparation), anxiety reduced both syntactic complexity and accuracy. In contrast, under task rehearsal (conscious preparation), anxiety had a positive effect on lexical complexity. Specifically, in the second performance, anxiety improved both accuracy and lexical complexity under task rehearsal and enhanced fluency and lexical complexity under task repetition. However, under task rehearsal, anxiety reduced syntactic complexity for both simple and complex tasks. Under task repetition, anxiety deteriorated lexical complexity, but only when the complex task was performed. Furthermore, task repetition outperformed task rehearsal in six out of eight measures: MLTU, DC/T, CN/T, EFC/C, Vocd, and WRDFRQmc. The cognitively complex task also produced better outcomes than the simple task across these six measures, as well as WMP. Performance improved on the second attempt across all measures and WMP.