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Your assessment is correct: the Community Edition does not impose any limits on data volume or development features, and you can scale and grow your deployment using standard features like replication and sharding.
If your non-profit organisation has limited resources, I would strongly consider using a managed service like MongoDB Atlas. Atlas includes security best practices and makes it easy to monitor, backup, and scale a deployment without having a dedicated DBA or extensive experience. There are also some Atlas features like temporary users that might be helpful for giving volunteers access for a limited period of time.
You also have the option of a self-managed deployment, but would have to factor in the time and availability of your volunteers for monitoring, securing, and maintaining the deployment.
Regards,
Stennie