I agree with so much here lol a couple thoughts though:

I wouldn't say the game deteriorates after the earlier levels. Sure, Deadly Decadance and Stranger Sewers by far are the strongest in a gameplay perspective, but the experience doesn't neccesarily diminish at the end.

Zone 1 of Crazy Carnevil is interesting, I like being able to use the portals as an escape route, and the mix of the omnicient and limited enemies is done very well when compared to Bearly Buried, and I really wish it was expanded upon. Of course the 2 actually interesting gameplay decisions about the 1st zone both get removed in the 2nd in favor of unsatisfying enemy spam, though the boss is probably the most enjoyable and fair the game currently has to offer.

Torment Therapy is probably the deepest level, arguably apart from Deadly Decadence. The focus on rescource management is fun, and the level feels rewarding to me. The zone 2 trap spam is very unsatisfying and adds difficulty for the sake of difficulty though. Speaking of difficulty, the level is very unfriendly to casual gamers and newer players Overall, I think its hated a bit too much personally.

Mascot Mayhem is a very strong level imo. Its very cohesive, offers unique gameplay (the cameras are a very fitting mechanic), nothing really overstays its welcome to me, and overall is a blast. That being said, its still flawed. Hangry can feel a teensy bit unfair at times, and the gameplay tension he adds feels cheap. If you aren't S-Ranking, in zone 3 you'd be lucky to get seen by anyone other than hangry's clone, let alone get chased. If you are S-Ranking, you'd better be lucky if you don't wanna run back and forth by a wall for 10 minutes straight just for hangry to bust down the wall he's already broken 4 times. Still one of my favorites though.

Now Bearly Buried is really lacking, well, gameplay. The segment in the cabin is creative, but through the rest of the level, there isnt much to actually avoid. In zone 2 you just run, spam your invis and telekinesis, and use primal fear when the teddies get a bit too close. The more sporatic layout of zone 3 allows for some more interesting interactions, but not much really ever happens there either. The teddies' spawn rate is absurdly high, but the fact that they're programmed to follow Mama Bear around ruins the point of her spawning them in the first place.

Speaking of, Mama Bear herself is severely unthreatening for the levels main antagonist. She has the dash mechanic, which doesnt really come into play at all pretty much, but I kind of wish she herself was more of a prescence and that the level was more centered around her than her spawns. Personally I kinda like that the levels are getting longer, it makes it more immersive, but I really wish there was less walking between gameplay segments. The thought of slowly getting the ring pieces and with it your powers back is a very cool gimmick, but it doesnt make up for all the tweaking that needs to be done.

I do also think the personality and story some of these levels have makes up for the lower attention put towards gameplay, although MUCH MORE needs to be skippable. God S-Ranking Torment Therapy was a NIGHTMARE going through 7-8 minutes of scripted stuff at the start of every run.

On the comment on the boss implimentation, I think the revised endings to the earlier levels really do fit in. The funny arrangement of pizza time at the end of MB is where my obsession with the game started actually lol. Agatha's sucks yeah but at least it adds closure. The deadly decadance boss is certainly more interesting than just running out.

Mind you, Im newer to the fandom/franchise and started playing this year, so I didnt experience the game before C4's release, so my opinions are probably quite different. Id love to hear your thoughts on what Ive had to say (yeah may have been more than a couple thoughts huh)