Context: Developer experience (DX) plays a key role in developers' performance and their continued involvement in a software ecosystem (SECO) platform. While researchers and practitioners have recognized several factors affecting DX in SECO platforms, a clear roadmap of the most influential factors is still missing. This is particularly important given the direct impact on developers' interest in SECO and their ongoing engagement with the common technological platform. Goal: This work aims to identify key DX factors and understand how they influence third-party developers' decisions to adopt and keep contributing to a SECO. Methods: We conducted a systematic mapping study (SMS), analyzing 29 studies to assess the state-of-the-art of DX in SECO. Additionally, we conducted a Delphi study to evaluate the influence of 27 DX factors (identified in our SMS) from the perspective of 21 third-party developers to adopt and keep contributing to a SECO. Results: The factors that most strongly influence developers' adoption and ongoing contributions to a SECO are: financial costs for using the platform, desired technical resources for development, low barriers to entry into the applications market, and more financial gains. Conclusion: DX is essential for the success and sustainability of SECO. Our set of DX factors provides valuable insights and recommendations for researchers and practitioners to address key DX concerns from the perspective of third-party developers.