Anyone actually identifying content gaps successfully?

yeah. content gap in ahrefs or keyword gap in semrush does most of the work - your domain plus 2-3 competitors, and it lists what they rank for that you don't. the hard part is filtering, most of that list is junk or off-topic so you have to go through it by hand. the other source is search console: queries where you already get impressions but have no page really about them. those are the easiest gaps to fill.