Hey Jay!
My name is David Black and I'm the Director of Customer Success here at SEMrush. I see you mentioned our service so I wanted to throw this out there... have you tried the new SEO Ideas tool within our Projects Suite? I would really encourage you to try it if you haven't already.
Essentially, we're doing exactly as you described. Based off of the SERPs collected within your Position Tracking campaign, we scan the content of the pages that rank within the top 10 for your target keywords and pull together a list of related keyword suggestions found on these pages and analyze how frequently they use them. This is going to help enrich your own content or find new opportunities.
It also pulls together ideas for backlinks and onsite optimization. The best part is that it does it on the fly as opposed to our regular keyword analytics reports. It's pretty awesome. You can check out an article from our User Manual that should help you get started - http://www.semrush.com/manual/what-is-seo-ideas/
I hope this helps!
