The finger has a stepper, it also has three other sensors connected to it. So composition.
I notice in the code you have protected: what is the purpose of this?
I gather that keeping the composition would mean that I can still have Thumb.stepper.moveTo(90); which would be ideal as like you said it is more futureproof.
However in your code, the constructor for the stepper doesn't have it being set as DRIVER? Is this not required? From what I gather AccelStepper does not default to DRIVER mode?
Finally, in terms of stepPin and dirPin I don't need them if there is a better way around? They are just used for setting up the stepper object.