SuttaCentral
How sure are we what Buddha meant? You can interpret it as wrongly thinking what is the unconditioned. (Attate) And wrongly thinking what is Atman.
For example replace Self for Atman.
Mendicants, whatever ascetics and brahmins regard various kinds of things as Atman, all regard the five grasping aggregates, or one of them.
Meaning it’s when you go beyond that only can there be Atman. Because it’s free from I am.
Even with the self in etc I find we can’t be sure.
Regarding the notion"I am", the sutta’s teach that this exist in relation to the 5 khandha’s.
For example…there are thoughts present…and because of that…you have a sense of “I exist”.
It is like the presence of thoughts confirms unconsciously “I am”
The same for feelings, tactile sensations, memories, emotions, longings, smells, odours, visuals etc.
In short, the five aggregates.
If you see this, you can see there is a tension in it too. Because, what if mind becomes so stilled that thoughts cease? If your inner scheme: “there are thoughts so I exist” is very strong, than your mind will react with fear when thoughts cease. Because you and thoughts are very strongly connected.
What would happen if suddenly your usual perception of a body, all tactile sensations, would cease?
What happens? Probably you will be alarmed because at that moment you instinctively think…oops, i do not exist anymore! Alarm. The mind is alarmed.
There is much tension in the mind because it is extremely attached to what is usual. The unusual is very problematic for a mind in which there is not much wisdom yet.
I am just starting to see this.
It is like we live in mirror-like-world. As long as we experience the reflections we are used to experience (tactile feeling, vedana’s, sanna, sankhara, vinnana’s) there is unconsciously a sense of safety. All is oke! But this also comes with a sense of unsafety when unusual things happen. For example, when thoughts or bodily formations cease. And then you start to say a very deep mechanisme for how suffering arises based on the conceit I am (asmi mana).
What do you mean here?
Out of topic. (Mostly spiritualist in recreational medicine experience what you said) that’s why I believe tackling fear itself is really important. As we can see Buddha did. He looked for the reason he fears. And reason some spiritualist says they couldn’t go further than the fear to truly let go of feeling they are loosing their self. Is mainly because not knowing why there is fear.
That’s why Buddha teach mainly about the views we need to let go. So we are more free before the big moment.
Let’s say in place of Atman. Think Buddha taught what Vedanta has as unconditioned part of Atman. (Attate)
Then when Buddha says about impermanent things being non self he could be saying impermanent things are the conditioned self only. He teaches of the unborn, unconditioned still. So meaning there is something else. That’s free from self but you still can reach it. 
but without the I am attachment towards the internal and external world.
I very much agree with you. There is a fear of loosing ourselves. It needs investigation. To be understood.
Yes. That is arahattamagga/phala. I nowadays (disclaimer) think words like unmade, unbecome, unconditioned, unproduced (Udana 8.3) are maybe used by the Buddha to illustrate and justify that there is a limit to our efforts. The pure heart is not made. It cannot be made by effort and does not have to. This we must understand and see.
One can develop the body by effort and become a body-builder and the result is made. But the pure nature of the mind is not made by us. It is our birthright, like you suggest somewhere else. I belief that is true.
There is something that cannot be shaped. And, i belief, that is the core of Buddha-Dhamma. Our bad deeds do not make it worse. Our good deeds do not make it better. It is beyond that.
Wish you the best.