You are outraged at the sentence. Which is fair enough. I have explained why the three reasons you gave for a lower sentence are perfectly legally valid and, in my opinion, correct. I am not making excuses for anything. I am saying it is right. Your mileage may vary. My comment is that if you had experience of what the victim can go through you would see the logic more clearly why credit is given for an early guilty plea, even if you don't agree with it. I am not, to be absolutely clear, saying you have to have legal experience to make a comment.
As I have said twice, the sentencing remarks and actual facts of the two cases need to be read to understand the comparison in sentencing. Taking the headline three reasons without any context isn't helpful.
There is no minimum custodial sentence for rape in the laws of England and Wales. Like none. The guidelines would expect a minimum of 4-5 years, and obviously, a life sentence is also available at the other extreme. So, there is actually a very wide range available to the court. Because there are different levels of the offence.