Rhyming dictionaries have existed for a very long time: at least back to 1775[1].

Not that an app wouldn't be useful to someone, but if they need rhymes today, bookshops (and websites) have an MVP already.

[1]: https://en.wikipedia.org/wiki/Walker%27s_Rhyming_Dictionary

He meant something like "Rhymes would be cool, but that's not what I'm telling you. I want to see ideas related to what I'm writing about."

1775 is not "a very long time" ago by most definitions.

Rhyme dictionaries are significantly older than that. https://en.wikipedia.org/wiki/Qieyun

(Ironically, old Chinese rhyme dictionaries have considerable scholarly importance because they allow us to determine whether old Chinese text probably rhymed, a task the text itself does not help with. They are less valuable for the other direction of retrieving a list of rhymes for a particular character.)