Due to its high complexity of involving not only researchers, but also users and other stakeholders, participatory research can benefit from exchanging methods, experiences, and lessons learned across project boundaries. To support such collaboration, we developed the ParticipationHub, a digital communication and collaboration platform for a funding program on participatory design with informal caregivers. Based on workshops, continuous feedback, and a formative evaluation, we reflect on the platform's implementation and use over the course of a 4-year funding period. Our findings indicate that while the Hub successfully supported coordination, knowledge sharing, and methodological exchange, its long-term adoption was shaped less by technical functionality than by social, organizational, and institutional factors. We discuss enabling and disabling factors for sustained participation and derive lessons learned for the design and operation of collaborative platforms in research funding programs.