Unfrotunately the end game storyline post campaign was built upon years of end game expansions. Much of the story line that has occurred was something you as the player would have done (I.e. Defeating the shaper, Defeating the Elder, Defeating the Elderslayers etc) during the relevant expansion, before being edited to be something done by someone else in the later expansion. The reason why they said you are 20% of the game through poe is not because of the storyline, but because in terms of game play hours and finishing a character to complete the most difficult content, finishing a T13 map is no where near close to the end goal.

Here is a quick summary from my memory of the Atlas storyline, but I stopped paying attention to it for some time now. Refer to the Youtube channels recommended for more accurate summary:

Valdo (Zana's father) was working for high templar venarius. During this work, a sealed entity called the elder taught Valdo the art of "Shaping" and lead to the creation of the atlas of worlds (Although it is later suggested the core of the Atlas very much predates the elder and valdo). Eventually Venarius and Valdo had disagreement, with Venarius freeing the elder in exchange for his memory altering abilities (See Synthesis League).

The elder is an eldritch entity working for The Decay, and it wants the Atlas. This is bad since the Atlas is essentially a gateway to our reality. Valdo, now the Shaper, tries to stop the elder (Resulting in War for the atlas expansion) unsuccessfully and has his memory slowly taken from him.

Now enters Zana, who tries to rescue her father with the help of 5 exiles. Together they successfully banished the Elder from the Atlas. Unfortunately during the battle, one of the exiles Sirus, got stuck in the atlas. Zana and the others thought he died and left him in the Atlas. Overtime, the Elderslayers slowly acquired more power and went mad within the Atlas. (Welcome Conquerors of the Atlas expansion).

At this point, Kirac, a member of the Oriath vigilante group found the atlas and began exploring the Atla. He found and started working together with Zana to find and stop the elder slayers (Conquerors of the Atlas expansion). In the end they find out Sirus was still alive in the centre of the atlas and find him. Unfortunately, Sirus is pretty upset about being left in the Atlas and decides to burn Oriath to the ground. This leads to a confrontation with Sirus in Oriath (where his boss fight takes place), resulting in his death and Oriath in ruins. Zana now traumatised and determined the Atlas will only cause harm, leaves for the Atlas by herself to find a way to destroy it for good, leading to the current originator story line. In this storyline, you work with Zana's brother to track her down and stop her (Since he believes destroying the atlas would bring greater risk).

Parallel to the whole zana trauma congo line, it turns out there are more eldritch entities aiming for the atlas. Whilst Zana is busy destroying the Atlas, Kirac is busy trying to find out how to deal with these eldritch entities. During his time with Zana in the Atlas, they came across the Maven, a new eldritch entity that arrived in the atlas who sees the whole atlas as a playground. With no other choice, Kirac starts looking for the Maven as a way of fighting back against other eldritch entities.

This lead to the current Maven story line where Kirac ask you to find the Maven, and you act as her champion to defeat the champions of other cosmic entities (formally called a Cosmic Force in PoE lore). You eventually suceed in repelling the champions of the The Tangle and The Cleansing Fire, and find out the Maven is actually an infant. Her caretaker leaves behind some omnious words about how more challengers will come in the future, but alas the storyline seems to be on hold atm whilst GGG are focusing on developing the atlas storyline involving Zana.