Personally I just use a google sheet with a separate-sub sheet for each category of content.

Then, on that sheet, I simply have two columns: keywords and # of searches for those keywords. I also keep a column of notes (eg. if the competition for a given keyword is particularly fierce) and then also, a few columns away, I keep a list of content ideas for that category.

Finally, I have a “To Do” sheet at the end. Whenever I run low on to do’s I just scroll through the category sheets looking for ones with high search numbers and enough content ideas.

No idea if this is the best way at all, just what I do.