Endwalker's story had me, but lost me. Ethical issues especially (spoilers)

[Iscah]

The nature of G'raha's time travel, that is to say going through the interdimensional rift, would suggest from the start that we're dealing with parallel timelines and not divergent timelines.

The rift does not take G'raha from one timeline to another. It takes him from the Source to the First, both of which exist within a single timeline.

Having originated from copies of the sundered original world, the Source and First (and all the other shards) superficially look like different timelines, but they are not. They are physically linked and the whole basis of the Eighth Calamity is that they are rejoined to each other.

The plot of Shadowbringers is entirely about G'raha coming back from the future of the Source-and-First where things went badly for both worlds together, and instead creating a new branch of time where both worlds are saved together.

It's correct that the other instances of time travel are causal loops, but that just tells us that it's possible for causal loops to form, while G'raha demonstrates that it is also possible to break that causal loop if you change the right thing in the past - and the short story An Unpromised Future confirms that its outcome is two lasting halves of a split timeline.

Even back in Alexander, it's acknowledged that there was a risk of time being altered. It just didn't happen that time because we stopped the Illuminati before they could do so. For the first part of the story they were deliberately repeating the "prophesised" events recorded in the journal, so they weren't interested in doing anything that would break the loop at that point.