Sustaining changes in teachers' practices is a challenge that determines the success of curricular reforms, from which Digital Education (DE) is not exempt. As the literature on sustainability is considered "scarce" and "scattered", long-term studies modelling the factors impacting teachers' sustained uptake of DE pedagogical content are lacking. Thus, we investigate whether and how 287 in-service teachers sustained a primary school DE curricular reform over a year after they completed their two-year DE professional development program. We model the sustainability of the reform through Structural Equation Modelling, and identify critical sustainability-factors. The validated Sustainable Adoption of Digital Education (SADE) model confirms that sustainability in the fourth year of the reform depends on perceived usefulness of teaching the new content, ease of implementation, and access to sufficient support in schools. Such factors should thus be evaluated, accounted for in the implementation phase of the reform, and sustained over time. The findings confirm that the DE curricular reform model contributes to positive self-efficacy to teach DE, provides sufficient in-school support, and promotes increasing adoption over time. However, as teachers' practices have not yet stabilised, and teachers may still adopt more to cover the breadth of DE-concepts, it is important to remain attentive to remaining sustainability barriers: lack of time, effort required to teach DE with teachers preferring to delegate, and lack of student-learning evidence, the latter being a significant challenge to address in the literature. These barriers must therefore be jointly addressed by researchers and practitioners in the field in order to promote the sustainability of the reform.