Will try adding more solids.
Yes that is the main problem, becouse there is a lot of hazelnut paste, which is fluid. I have found on one website very similar recipe, and on a picture it looks thicker. I tried contacting them, but with no success. https://www.lesdelicesdecaro.com/la-pate-a-tartiner/#comment-11438
I am making gianduja myself: 2:1 (milk chocolate:hazelnut paste). I am doing exactly what you described! It is quite liquid at the start, so I am hoping that it will crystalize, maybe it thickens up a little bit, but not as much as I would hope.