I mostly used the xbox controller when tuning it. With the analog sticks you have much more ability to make slight adjustments to the speed. I think the way that the axi work in Unity for binary input is that the float-value slowly goes up or down as long as the button is pressed. While I can change the rate at which that float is increased, it would still be a completely different feel than pressing the stick slightly; you can't for instance just press keyboard-button down half-way to go half speed.

I also considered making Z-movement more active (like using 'w' and 's' for inwards and outwards), but it adds a level of complexity that I found undesirable. Part of the sensibility I was going for here is the type of sculptural motion of a potter's wheel. On the potter's wheel, certain dimensional planes of movement happen easier than others. For instance, pulling the clay up and expanding the shape horizontally are much more easy than making the shape shorter or thinner. While those types of movement are possible, it really requires you to switch the way you think about what you are doing.
The thing I like about having presets of limited movement is that the shapes you end up creating borrow a lot of beauty from the resulting symmetry and consistency of the limited and passive motions. In other words, if the drift-away/drift-toward movement has a consistency while the player arbitrarily goes up and down and side to side, the resulting shape is more of a coil than a scribble. It's kinda like how you can't fuck up if you only play the black keys of the piano.