Large university classes with diverse student cohorts pose persistent challenges for maintaining engagement, delivering effective assessment, and providing timely feedback at scale. This paper introduces a participatory approach to co-creating constructively aligned formative assessments with students as a direct response to these challenges, demonstrated through a very large, online, and asynchronous Data Literacy and Analytics (DLA) module across 15 programs at Dublin City University. The Formative Assessments Co-Created with Students (FACCtS) framework guided the collaborative design of 500 interactive formative assessment questions with immediate feedback, involving students who had completed the module to ensure learner-aligned, inclusive design grounded in Universal Design for Learning (UDL). Reaching over 2,000 students annually, the approach demonstrates that co-creation is not only pedagogically sound but scalable across large, diverse cohorts. Post-implementation evaluation showed improvements in student engagement, metacognitive awareness, and summative assessment performance. The study offers a transferable, research-informed model for addressing the core challenges of engagement, assessment, and feedback that are amplified in large, online, asynchronous higher education contexts.