Atlast....
Everyone, Upanishads are only for Liberation, by attaining "Sat-chit-Ananda" or Purusha or Brahman or Atman or GOD or whatever name one calls.
The Maha-vakhiyas are MANTHRAS, to be repeated for enlightenment and release from Bondage, and it is said so in "Sukra Rahasya Upanishad"
The Enlightened Gurus, teach one thing to one disciple, and another thing to another disciple, based on his perceptions,
-like to One,
"I am the world, I am the sun,moon, I am the space,air,fire,water,sky, I am the body,mind,intellect, I am all-pervading, etc.."
- and to another,
"I am neither world nor body etc.."
- and to some other,
"I am neither being nor non-being neither bondage nor liberation"
- and to few,
"I am existence-consciousness-bliss, and other all mithya"
Like this, based on the disciple's nature, Manthras are said by the Guru, to be perceived for Liberation. It is considered as Knowledge for repeating as a Manthra only for Liberation, to attain the Purusha dwelling in HEART.
Every upanishad is to put an end to wordly pleasures, and prefer liberation by sticking to one manthra uttered by GURU, and concentrating on that manthra always to reach the "Supreme".
"I am THAT" is a manthra uttered to merge with "that", in what form one thinks 'that' as. If one thinks "THAT" as Swarga - he attains swarga and falls again to world in time. If one thinks "THAT" as 'Sat-chit-ananda", then he reaches the supreme abode, and rebirth comes to an end.
"AUM" is Manthra, repeated again and again by concentrating as "ONE" brings liberation from three states.
In Muktika Upanishad too, it is been said, Mandukya is enough for Knowledge to attain my abode. If knowledge is not got by Mandukya, then other Upanishads, to reach HIM, the BLISS.
All Manthras/words/images/form is uttered for not knowledge but for IDENTIFICATION as "One with THAT" for liberation.
The Knowledge is, "Everything is One self, Ishvara-Jiva-Prakriti is all ONE. Non-duality or Advaitam, is the END, where the combining of Purusha and Prakriti comes to an end and falls of, whole universe disappears, TOTAL PEACE, or nothingness as Buddha said. All this Prakriti (body,mind,intellect) is Karmic - so one should cut off his longing to these pleasures, which covers the BLISS behind as the whole," and by knowing that, one's pleasure from worldly things should be put off by the Manthra uttered by his GURU, and attain THE SUPREME ONE - Ishvara, who is the highest form of BRAHMAN, who is dwelling in the HEART of everyone/everything, full of BLISS. It is the Knowledge imparted to Nachiketa too in Katha Upanishad.
The Jnana in Jnana Yoga is, "Attachment to changing is the cause of pleasure-pain-sufferings, and detachment is liberation", by repeating manthras like "I am neither attached nor detached, neither bondage nor liberation, chit-ananda rupah -sivam I am", to discriminate and be detached and one will come to feel the BLISS and abiding in it continuously by the manthra.
This Knowledge of all Upanishads is uttered in chapter 13, verse 5-12 of Bhagavad Gita. Also discrimination between Prakriti and Purusha, Kshetra and Kshetrajna - and this is said in same chapter 13, verse 13 to 35 end, and this is to reach his destination.
https://www.holy-bhagavad-gita.org/chapter/13
Also, here,
https://www.holy-bhagavad-gita.org/chapter/2 , 2.54 to 2.76 end.
Also, here,
https://www.holy-bhagavad-gita.org/chapter/7 , this whole chapter is full of Knowledge.
One reaches the Supreme Purusha, Sat-chit-ananda and from there, when universe disappears, dissolves in Non-duality, in ETERNAL PEACE and doesn't have sense of "I AM". Again from non-duality to duality, by merging of Purusha and Prakriti, universe comes and the jivas who were not liberated in previous universe, takes birth and it goes on endlessly.
These Manthras are the Powerful Thorns, used for removing the Karmic Thorn, and both is thrown when when is liberated, as said by Ramakrishna.
Devotion/ I am Brahman (or any manthras even buddhists,etc..)/ Kriyas / Karma-yoga, or anything doesn't matter, but to put away the worldly bondage/thoughts/pleasures - to end sufferings - they are the MEANS.
Om Shanthi Shanthi Shanthi.