When I already have determined a “main” or “core” keyword for my site or page, what’s the best way to choose keywords that are related to that term/phrase?
The best way to explore and choose related keywords to expand past your “core”/”main” keyword is to combine real world input and feedback from your client, customer, support and sales teams with available SEO data for what terms and phrases Google understands as relevant to your core phrase.
With our SEO forecasts we use a process using DataForSEO and it’s scraped set of data to ask “who ranks for the main phrase”. We then take the top 10 URLs from that resulting SERP and ask what 100 keywords THOSE pages ALSO rank for, and usually get 300-500ish keywords. The best part is that based on the “overlap” of how many of those URLS also ranked for that phrase, we have a sort of “score” for how relevant Google thinks those phrases are naturally.
When you comb through those results, eliminating extraneous phrases or items only competitors offer, you end up with a really relevant set of keywords to target with additional pages, posts or to optimize existing content that matches that intent. Of course, don’t forget to visit the SERP for those terms, to get a “lay of the land”!