why i quit before reaching endgame (feedback from a new player)

[Seraphor]

It's worth breaking your time up by revisiting the older content. Coils, Alexander, Crystal Tower, Mhach, maybe even PvP, in order to avoid burnout. The good thing is that all content is played, it's not forgotten about just because it's not the latest expansion content. (with the exception of Diadem)
Although some of it, like Coils, may require you to find a good FC or LS to coordinate with other players for it.

You have to keep in mind that you're essentially playing through three expansions worth of story heavy content, this isn't like WoW where you just start at the latest expansion, this is all one cohesive story.

You're not going to just catch up to Shadowbringers at this late stage unless you put up with the slog of story content, and unless you skip all side content.
So don't rush it, and take it at its own pace.