>focuses too much on the fully developed cast
Not sure about that one. They literally tried to set up new characters out of Starlight and then the Pillars, but they literally can't write for them. Starlight gets rewarded for saving the world and reforming all changelings instantly in the end of season 6 and then goes on to condemn a changeling in season 7. The Pillars were all thrown away, getting a couple of voiced cameos in a couple season 8 episodes and the remainder of appearances are silent and brief. They then create yet another team of creatures which have nearly zero connections to the main cast with different backgrounds, cultures, and built in hatred to pony culture as well, and they give exactly one episode to explore any of it. The writers just suck, even when they try to tear down the original to build their own thing through characters designed for it.

>panders to the older audience
For every example of pandering to the adults, there are a dozen examples of the opposite. The battle at the end of season 5 was downgraded from season 4, clearly returning to more child friendly content. From then on, there was never a single battle even approaching a similar scale until the series finale. The episode that is designed to cater to the adult desire to see the apple family history is immediately followed by the show calling out adult fans with adult criticism and concluding that the only audience they should cater to is the children. The only scary element of the show, the changelings, were turned non-hostile and completely altered in appearance.

>good episodes especially in season 7
OP has been brainwashed by the one good episode in season 7, the family history one. That one episode excused every bad decision from the episode that flat out attacks fans to the worst set up of new characters that were thrown away and the worst villain of the series paired with extensive gaslighting to make said villain the one in the right. I feel like no one really gets just how bad season 7 was.

>some good episodes in the final two seasons
I really need OP to be specific because I can recall only one decent episode in the whole of the two seasons. The villain team up episode to retrieve the magic bell was the most fun after so many failures with building friendship, character exploration, and motivation building to explain how the villains will even get power to be a threat. Everything else was a rushed set up for the diversity school, resurrected episodes from the trash bin, and a disappointing final conflict utilizing bait and switch with a depressing epilogue to top it off.

Besides this, the grouping is still very basic in understanding. There was still quite the difference between seasons 1 and 2 as well as 3 and 4 in story design. Yes, everything got worse as it went along, but the divisions that are shown doesn't explain the extent of the decline and how it was a feedback loop of bad writers making decisions creating yet more bad decisions for even worse writers.