Digital health tools, such as radiotherapy preparation apps, can support person-centred care, though their development is shaped by evolving complexity. This study examined how perceived complexity changed during the development of the Deep Inspiration Breath Hold Preparation App and assessed the utility of the NASSS-CAT. A longitudinal observational study was conducted, with complexity assessed at three development stages using the NASSS-CAT. Stakeholders from clinical, academic, and industry settings completed individual assessments followed by group discussions. Quantitative data were analyzed descriptively; qualitative data provided contextual understanding. Perceived complexity varied by domain and over time. Initially, technical complexity was rated highest. Over time, total complexity declined, especially in strategic and operational domains, reflecting greater integration and shared understanding. The NASSS-CAT supported stakeholder reflection and adaptive decision-making. Integrating complexity-informed evaluation into development processes may improve the scalability, sustainability, and person-centredness of digital health interventions.