Сначала примените layoutMargins с UIEdgeInsets, чтобы иметь интервал, затем для каждой кнопки в stackView измените cornerRadius
stackView.layoutMargins = UIEdgeInsets(top: 0, left: 20, bottom: 0, right: 20) // change 20 with your value.
// then for each of subviews in stackview, if it is a button, apply corner radius
stackView.subviews.forEach { view in
if view is UIButton {
view.layer.cornerRadius = 5 // replace it with your value.
}
}