Почему не Тхеравада

Conditioned (samsara) and unconditioned (nirvana) both are inexistent, because they are just imaginative things.
И как вы можете применить это на практике, в своей жизни? Как это связано с прекращением дукхи?

Подобно этому нет страдания, нет возникновения страдания, нет прекращения страдания, нет пути прекращения страдания, нет мудрости, нет достигнутого и нет недостигнутого. О Шарипутра! В соответствии с этим для бодхисатв нет достижения; опираясь на эту глубокую запредельную мудрость и пребывая в ней, они не имеют препятствий и омрачённости, и, уйдя от заблуждений, они достигнут полного освобождения, нирваны.

In general, the question is: this suchness, which on the one hand is empty, and on the other hand is not empty, because it has qualities, it is conditioned, does this suchness have a cause, or is it not conditioned, has no cause? Is it causal or causeless?

You can’t say that true nature “has” this or that anything, because it isn’t some kind of thing that can possess an aspect or quality.

You can’t even really say that true nature “is” either this way or that way, because as soon as you do that, you are stuck in duality.

And if you only talk about the true nature “of” ...what? Of mind? Of being? Of awareness? That doesn’t work either.

That’s why zen teachings often refer to the unborn mind, or your face before your parents were born. If you try to figure out whether tathagatagharba arises with conditions or without conditions, you get stuck right there because it neither arises nor subsides. It’s just there.

Whether or not recognizing it depends on causes, that’s a different issue. Getting you to where you can recognize it depends on causes. It’s like the apple ripening on the tree. But what makes the apple suddenly fall? It lets go. The causes that ripened it are over and gone. Yeah, you can say gravity or a strong breeze, but those things only influence how it falls.

About one of the aspects of suchness: "The other is that it is truly nonempty (asunya), for its essence itself is endowed with undefiled and excellent qualities" (A treatise on the Awakening of Faith in the Mahayana).

It is said that suchness at the same time, is not empty, is not empty - ashunya. The same thing is implied by zhentong.

I think your question is really about the Absolute and the Relative. It makes no sense to talk about cause without effect. In addition, it is assumed that some cause is not same as its effect. The cause and effect are therefore relative; they are conventionally considered as different (but they are neither same nor different in Reality). The Absolute, by definition, is all-inclusive; it is therefore automatically non-dual (not-two); it automatically contains the relative. The Absolute cannot be caused by anything "outside" it. The Absolute cannot became the effect of some "outside" cause; it cannot be conditioned by something "outside" the Absolute, because then it would be relative to that "something outside" and therefore different, and that would violate its definition as the all-inclusive Absolute.

During typical 7-day Zen Sesshin in the West, there are recited various chants (dharanis). One of them is the dharani about "The Identity of Relative and Absolute", which I give below. I hope it helps to answer your question. The text of the dharani is taken from:
https://zencare.org/chants/

The mind of the Great Sage of India was intimately conveyed from West to East.
Among human beings are wise ones and fools,
But in the Way there is no northern and southern Ancestor.
The subtle source is clear and bright;
The tributary streams flow through the darkness.
To be attached to things is illusion;
To encounter the absolute is not yet enlightenment.
Each and all the subjective and objective spheres are related,
and at the same time independent.
Related and yet working differently.
Though each keeps its own place,
form makes the character and appearance different.
Sounds distinguish comfort and discomfort.
The dark makes all words one; the brightness distinguishes good and bad phrases.
The four elements return to their own nature as a child to its mother.
Fire is hot, wind moves, water is wet, earth hard.
Eyes see, ears hear, nose smells, tongue tastes the salt and sour.
Each is independent of the other.
Cause and effect must return to the great reality.
The words high and low are used relatively.
Within light there is darkness, but do not try to understand that darkness.
Within darkness there is light, but do not look for that light
Light and darkness are a pair, like the foot before and the foot behind in walking.
Each thing has its own intrinsic value and is related to everything else in function and position.
Ordinary life fits the absolute as a box and its lid.
The absolute works together with the relative like two arrows meeting in mid-air.
Reading words you should grasp the great reality.
Do not judge by any standards.
If you do not see the Way,
You do not see it even as you walk on it.
When you walk the Way it is not near, it is not far.
If you are deluded, you are mountains and rivers away from it.
I respectfully say to those who wish to be enlightened
Do not waste your time by night or day!

Позднее - это систематизация ранних идей, более глубокое их осмысление.

Antaradhana, систематизация, это не совершенствование, это не дорабатывание. Это осмысление, вот из определения систематизации - это мыслительная деятельность, в ходе которой исследуемые объекты организуются в некую систему. Один из основных видов систематизации — классификация.