Sorry to hear that :(
I get what some of what you're saying here, but I also feel a couple of these complaints are misguided.
For one thing, the Redux team has never intentionally tried to make Redux "the main state management tool for React". We've never deliberately sold it that way, and goodness knows the React team has said "React is the only official state management tool for React". Redux got popular because it solved the problems people were dealing with when it came out, better than the existing tools in the ecosystem, at a time when React was taking off. We've done our best to clarify in the docs when it does actually make sense to use Redux.
Redux Toolkit is still Redux. You're still writing reducers, creating a store, dispatching actions, and updating the UI based on state. RTK just allows you to do so with less code. So, the "Essentials" tutorial starts by introducing those terms, concepts, and data flow, because you need to understand those regardless of how you're writing Redux logic.
While Redux _is_ normally used with React, it's also UI-agnostic, hence the separation from React-Redux, and using it with other UI frameworks is a valid use case that we support. We had some long debates about whether to add these additional APIs into the Redux core [0] [1], and concluded that there was too much inertia around the existing Redux ecosystem to build those into the core `redux` package. As it is, even though RTK isn't required, we've had folks say they didn't like some of the opinions we've built into RTK (like using Immer for writing reducers). So, combining all three of those packages into a single super-package is not feasible at this point.
I've tossed around the idea of trying to merge the Redux core, React-Redux, and RTK docs into one site, but that would likely require merging all three libs into a monorepo. Not impossible, but not something that's likely to happen any time soon, if only because my own time is very limited and I have a lot of other docs work I need to focus on [2].
Which specific docs page are you referring to? You said "intermediate" and "advanced" - sounds like you're referring to the RTK docs site tutorials [3]? FWIW, I wrote those a year ago, and at the time the main Redux docs were frankly very out of date. I've spent much of this year rewriting the Redux core tutorials, as listed above, and those Redux tutorials are where I would direct anyone who's new to Redux or really wants to know about RTK. We do also have an open issue to discuss what tutorial content should be on the RTK docs site [4], and tbh I'm really not sure _what_ tutorial content should actually be on the RTK site now that the "Essentials" and "Fundamentals" tutorials exist in the core docs.
You're right that the actual Redux core itself isn't very complicated, but the biggest issue is trying to teach people to "think in Redux". In fact, it's kind of _because_ the core is so small that there's a lot of documentation needed - so much of this is actually in the patterns around how to use Redux effectively.
Anyway, if you can point to some more concrete concerns or have specific suggestions on how we can improve things, I'm very interested - please feel free to comment over in the issues.
[0] https://github.com/reduxjs/redux/issues/3321
[1] https://blog.isquaredsoftware.com/2019/10/redux-starter-kit-...
[2] https://github.com/reduxjs/redux/issues/3592
[3] https://redux-toolkit.js.org/tutorials/intermediate-tutorial
[4] https://github.com/reduxjs/redux-toolkit/issues/371