Self-reported attribution

You add one open question to your signup form, checkout or first call: "How did you hear about us?" People often name things software cannot see, like a podcast, a friend or a subreddit thread. Their answers fill gaps in your UTM parameters data.

With few customers, this is often the most useful attribution you have. Make the field a free text box, not a dropdown, and read every answer. Log it in your CRM and track which channel brought each deal.