EightWayController.cs

Vector3 inputVec = Vector3.zero;

if (Input.GetKey(KeyCode.UpArrow))
  inputVec += Vector3.up;
  
if (Input.GetKey(KeyCode.DownArrow))
  inputVec += Vector3.down;
  
if (Input.GetKey(KeyCode.LeftArrow))
  inputVec += Vector3.left;
  
if (Input.GetKey(KeyCode.RightArrow))
  inputVec += Vector3.right;
  
Vector3 inputMagSqr = inputVec.sqrMagnitude;
Vector3 inputDir = 
  inputMagSqr > Mathf.Epsilon 
    ? inputVec / Mathf.Sqrt(inputMagSqr) 
    : Vector3.zero;

Vector3 moveVec = speed * inputDir * Time.deltaTime;