You get a wrong impression and understanding with this one.
None of Buddhist schools accept 'the observer'.
Buddha nature is not the observer.
Rigpa is not the observer.
Citta that never dies is A disaster misunderstanding as well.
Regarding Buddha nature, buddhahood is not something you can create like creating a car. If you can fully realize there is no self in everything, you realize your buddhahood right now. Not after you die, but right now, this second.
Everything doesn't have self. There is no such thing as today I have self, then after following Buddha way I find this I and I destroy it. It is not like that. In this case, you can find the real Self and then you destroy that self. It is not like that.
Now, we think there is self. But if you search whether you are usin theravada method or Mahayana method or Vajrayana method, it is right now you find the answer that this self or I is unfindable. So, actually there is already no self since last year, last last year, yesterday, today, and even in the future. This no self is always no self. This is called Buddha nature, the no self. Because everyone without exception always doesn't have self or I, since beginningless time everyone is actually already have this Buddha nature.
If you can eliminate your belief regarding this I and 100% directly perceive no I or no self, your Buddha nature just shinning. You are Buddha straight away. But, if you cannot eliminate this no self, your Buddha nature is covered with your imagination of I, which you believe is there.
Buddha nature is unfindable, it is just a pointer referring to this state that right now you are already in the state of no I. But you don't believe it, you think there is I.
Rigpa and Buddha nature are same. Just different fancy name.
Only people who cannot accept there is nothing in spite these various appearances are there, they create citta or mind as the observer. But if you are critical, and you check it yourself, you won't find it.
There is a school based on this citta who assert citta is the main actor. But this school has been criticized almost 2000 years ago.
If we talk citta or mind only on the relative level AS IF you can find it, that is fine. But, if that citta or mind give you the impression that mind and citta is real and findable, then it is a big big mistake.
I am not here nor there.
I am not right nor wrong.
I do not exist neither non-exist.
I am not I nor non-I.
I am not in samsara nor nirvana.
To All Buddhas, I bow down for the teaching of emptiness. Thank You!