I've run 16 servo's simultaneously with DT_INTS.
Does this mean there's code out there that I can directly adapt?

Where would I find this?

explain with clear words what your device has to do :

from which inputs (w/ details ) to which outputs ( w/details ) ...
I have two inputs: start/stop button and a 'selector switch' which I already have working with that if statement you see above.

I have three outputs: two servos that I need to 'bounce' independently of each other. I need them to simulate the somewhat random movements that a passenger in a car would experience whilst driving. The third input is for vibration, this is just an unbalanced DC motor - np.

I also want to run a countdown timer so that it gets turned of after 20mins.

50% of the work is here ...
So now that my project is exactly half done, which way do I head?