I remember using that method but instead of having to place the points of the curve in the grid, I started out with a straight line and used the "Create Points" command. It would "split" every segment of a curve in two, creating additional points. So, a curve with N points would become a curve with (N*2)-1 points.
Then I would select every other point, displace them and then turn all the handles to smooth.
If we had scripting in Affinity Designer it would be too easy to implement a wavy curve from a selected curve. We could even define how many waves and the amplitude. Hint, hint ;-)
