Personally I did not hear a lot of movement in the 2 examples though I did enjoy listening to both of them. Creating motion from front to back is tricky which the 2 examples did show this to some degree. What Grassow Album(s) are the 2 tracks from?
What I am describing is from a mixing perspective, not programming a synth.
There seemed to be no automated panning of sounds in the tracks which is moving a sound across a horizontal axis, left to right and vice versa. This can be subtle of exaggerated.
Front to back motion is about understanding space by imagining, as I do, a stage as though a concert was being performed.....where do you place the instruments? For front to back motion it is about reverb / delay and also doubling of tracks. For example slightly pan a sound left in your Daw, with no reverb or very little, create a second track of the same sound and pan the sound hard to the right and add a judicious amount of reverb. Listen to the two sounds together and a long delayed tail is create but not from the first source. Adjust volume levels by creating and ADSR envelope using volume automation in your Daw as Seren suggested. Experiment! Listen!
I feel it is more useful to animate the mix and the way sounds are placed in the mix then laboring at a synth with various types of modulation. The sounds you create should be interesting to you, should inspire you as you build your drones but the real surprises occur when 2 different tracks come together and create a third. This in itself creates motion.
