Fiona,
Thank you for sending over your question and I hope I can provide you with the information you need. I do want to let you know that the Community ToolBox offers you a variety of resources to address your needs. For one, here is a model and resource for social action: /en/get-started. You will find specific resources that help you think about how to assess a problem/challenge and then how to develop a model for sustainability. I would encourage you to go through the steps, assess, plan, act, evaluate, and sustain and use the resources provided and tools. Since you mentioned "a sustainability intervention", I would like to point you directly to these resources: https://ctb.ku.edu/en/table-of-contents/sustain/long-term-sustainability/sustainability-strategies/mainh. The resources are very specific to strategies that help organizations and other entities think about strategies needed to sustain their work.
Thank you
