I'm admittedly overeager to build and am not very good at building financially sustainable projects, so I told myself I'd fund this feature with revenue from paying customers. ($0MRR right now)

In the meantime, you can use webhooks to push results into your own sentiment analysis flow. That's supported (and free).

Watson is very expensive after all.

Alternatively you can use an npm module to determine basic sentiment https://www.npmjs.com/package/sentiment