Just use the free version of ahrefs and get your keyword data from your own ranking website.
Saves time and money and means you can focus on content and links to begin with, while writing in the general direction of the niche, before you start to explore new topics, and see what you rank quickly for.
I've got tons of good keywords to write about from there. You can see easily just how many and what keywords each page ranks for, and expand on them if you want. It's a good reality principle to work with.