Why do you need email addresses at all? Like by all means have an opt-in mailing list to talk about updates. Have a discord server for support. But why force people to provide an email address other than the paywall?

• Try me out - Free - 3 queries per month
• Hobby - $0.99 per 100 queries. Must use within 1 day.
• Professional - $9 per 5,000 queries. Must use within 30 days.

I'm sorry this just isn't at all viable as a payment scheme. You are never going to gain a consistent userbase.

And realistically, if your compute requirements are high enough that this costing model is necessary then you've just learned why Netflix has never used the winning entry to it's recommender systems challenges in production, because they tend to be too computationally expensive to use in anger.

Why not look at a document retrieval system like Marqo https://github.com/marqo-ai/marqo ?

Put words into Marqo as keys with blobs of text of their usage and or written definitions as the documents.

It will probably retrieve relevant words from natural language queries well enough to be used for this purpose. And it will be cheap to evaluate.