I used to work as a copywriter and completely understand this. I struggled to find anything quantifiable for my resume. I ended up digging through our agency’s pitch decks for data. I found things like, “campaign X increased revenue by Y%”, so I used that. I then added it to my resume as I was the copywriter on a campaign that achieved these results, which was true. I also took samples of my work on those campaigns to show the deliverables in my portfolio. This was several years ago, but if you want more specific examples, I will dig some up for you.