Create UIView programmatically in Swift
You can move setting properties to lazy var declarations eg. 

lazy var centerStackView: UIStackView = {
  let stackView = UIStackView()
  stackView.translatesAutoresizingMaskIntoConstraints = false
  stackView.axis = .vertical
  stackView.distribution = .fillEqually
  return stackView
}()


This will divide your code nicely. In my opinion, extracting views to separate files has a sense only if they have a more complex structure and you want to reuse them in other places. What is important is keeping functions short and on point. Also, try to use more empty lines to visually divide logic blocks, eg. adding subview and setting constraints to it from another subview.