If you have $150/mo in your marketing budget;
https://www.semrush.com/api-analytics/
You get 1 million credits and the API data with a Guru Account -- you can buy it for 1 month only non-recurring.
SemRush gets detailed adwords type results -- for a price
Wikimedia has an api also to validate words
https://www.mediawiki.org/wiki/API:Main_page it is a 'free service' I have used it to validate words that are actually used -- 0 or 1.
Google trends is a good starting point also
https://www.google.com/trends/
Set your search settings on Google to 100 results and them copy the results and glean the keywords surrounding the word/phrase you searched -- understand semantics.