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When the code executes this line
stepperName.step(Steps);
it does exactly what you tell it to, ie it executes a number of steps. During the time that it is doing this nothing else can happen, hence the delay in execution of other code
One way to fix this would be to only execute a single step each time that the code gets to that point and to keep a note of how many times a single step has been executed and stopping when the required number of steps has been executed.
If you did this then the rest of the code will have a chance to run between steps. Note, however, that any delays in the other code will slow down the motor
Implementing this solution using the stepper library would be a good programming exercise for you.
There are other libraries that could do what you want without blocking the running of other code, including this one
Its author is active on the forum if you need help