I tend to agree with most of the criticisms about the arc.
Spoiler for Story up to 1.3:
They tried too hard to have a story that is developing on multiple fronts, over multiple timelines, with multiple overlapping factors/motivations, and it's just biting off more than they can chew. It makes the story seem very disjointed, like it was constantly hinting at "there's *so much here*!" but it's all in the periphery and delivered in these weird fragments. Although some of it seems interesting (the underlying theme of immortality and the drawbacks therein is pretty rich in potential, and they clearly put a lot of thought into the lore) but how much do you really care for any of the characters and what's going on in the plot?
The disconnect about Dan Shu is certainly the most blatant part, and the fact they changed it to be a side quest (rather than believing it was an integral part of the main quest) screams of the left-hand not knowing what the right-hand is doing. (Like two different writers working on the two parts in parallel and not sharing notes on how to make sure they align well together in the end, which they mostly don't. The side quest builds Dan Shu up to be an important person, and she ends up just being a forgettable extra in the main story.)
Even the timing of introducing banner characters was weird and chaotic. At least in Genshin Impact they've been pretty good about ensuring the characters on the banner are developed before putting them on the gacha (or at least in the same patch), but here the order makes very little sense. It's like it was chosen for marketing reasons "blindly" rather than in GI where they had the clear mandate that the story is what drives interest in the characters, or just that the story changed substantially in production for whatever reason and they couldn't shuffle the characters around.
On the other hand, as you point out, the Belobog arc was much simpler and self-contained. The plot develops in a logical, linear fashion, each point building on the last, and it has a very dramatic conclusion that ties up most of the loose ends. Obviously not perfect by any means, but it worked and is easy to follow and understand.
It does sort of feel like the first two arcs were developed in pre-production, but Luofu launched in an unfinished state and it became more about how they needed to keep having more maps, and more characters, and more "stuff" rather than a really tightly-knit plot. Perhaps they launched the game too soon, but thought they needed to strike while the iron was hot for some reason (to hit the gap between Genshin Impact regions, maybe?).
I guess if I had to make a suggestion, I probably would have kept the Stellaron Hunter stuff out of it more. I realize it's important to the overarching plot of the game, and they wanted to introduce the characters to the gacha, but I think it's still too early and all this is really not that satisfactory a way of introducing them anyway. The plot needed to be more focused and local. I would have centered it more on Dang Heng's development and kept him in the center of the plot rather than having him go off in a parallel journey. And as you suggested, they probably also should have developed Tingyun a lot more if they were going to pull this twist. Basically, make the story much smaller in scope, more focused, and more personal.
I think the reason why this gets particular criticism is indeed because it is returning to some of the similar issues that faced some of the early Genshin Impact archon quests, which they've gone a fair ways towards resolving over the years (not perfect still, but certainly improved). So people expect that the HSR team will apply those lessons learned already and not make the same mistakes, so in some ways it feels even worse. Plus, as many have said, HSR does not have the open world exploration aspect to keep people engaged, so in some ways it depends more on the story quests to keep people hooked. The audience who is going to keep playing the game just for things like SU and MoC are only a fraction of the audience.
Anyway, we'll see what happens next. Perhaps they'll find their footing after this chaotic start and over time things will stabilize. And I still think they have a lot of interesting ideas, but they're really struggling to nail the landing right now at least as far as story quests go.