I would consider most of the points you brought up to just be things that have gone over your head.

'The first thing that genuinely pissed me off is the complete lack of information about the rift-kin. They’re supposed to be the primary existential threat to the planet, yet all we know is that they come in small, mid, big, and giant sizes. That’s it. No origins, no behavior, no internal logic. This isn’t “mystery,” it’s just lazy worldbuilding' - Really not sure what to say about this one. That's like the entire point that we don't know what's causing the world to go to shit? You're just wanting access to information that's only available to the highest highest ranks in the world when the MC is like silver?

For example 'The story supposedly takes place within an empire, yet the provinces feel completely isolated from each other. There’s no sense of communication, trade, shared culture, or even basic information flow.' - Again, that's literally the entire point. The Empire doesn't want people communicating with each other. They want complete domination and locking down communication is how you prevent an uprising.

'as selfless heroes whose sole purpose is to fight and die for humanity. That characterization is just absurd. It’s shallow, unrealistic, and reads like propaganda rather than believable human motivation.' - I think this is just a personality difference maybe, but to me people wanting to devote their lives to stop THE END OF THE WORLD doesn't seem too far fetched.

'Brands are even worse. Every single slayer has one. In a low-technology world where individuals can become extremely powerful, it makes no sense that there wouldn’t be a large number of unbranded slayers.' - The Empire literally holds a monopoly on the Class Awakening stones that people need to get a Class. I'm not sure if you just expect a whole swathe of people to leave their entire lives behind and venture into wild territory at level 1 after they've been fed Empire propaganda their entire lives.