So I've been playing mania, loving it, but I'm not sure I actually think it's a better game than 2, 3, knuckles. It may be the nature of how it's built from recycled content and mechanics, the boss fights being painfully easy, the sphere collecting stages being painfully difficult (finally completed them all last night, each one has a twist to it and a 'take that' moment near the end, where I recall the 3/knuckles stages being more freeform and less stressful). It's spectacular, love the look, the drop dash speeds things up, but something about it doesn't feel quite as solid to me.
I don't know if it's an overabundance of speedy sections throwing you in all directions, maybe that combined with the easier difficulty in general promotes sloppy play?
The Studiopolis zone boss stands out to me as being a huge oversight. Why can you hit the boss with a simple jump from the beginning with no threat? You can wipe him out in seconds, where it seems he's supposed to be out of reach until the heat brings him down lower for you to reach him. Great boss battle ruined.
In general it just doesn't feel as tight, a bit of an interesting spinoff almost than feeling like a real sequel, but it's still the best Sonic game for me since Knuckles.
I have a tough time in general rating Sonic Mania in comparison to other Sonic games because it is more like a Majin Buu absorption of three other games instead of something mostly new. It also throws in somewhat random things like the Mean Bean Machine boss, obscure references for no other reason than a wink, the bosses overall and sometimes the level design. Like you just mentioned, there are some speedy sections that have dubious reasons behind them being where they are in the level.
A big thing for me that affects how I feel about this game is playing different versions of stages that I didn't necessarily care for, I'm actually surprised how many of them I was indifferent to, like Green Hill Zone, Chemical Plant, Hydrocity, I suppose my indifference towards them stems from the fact that they are old news, over 20 years old.
Then there are ones that I don't care for at all and their music is kind of annoying because it reminds me of Sonic & Knuckles, which has more stages that I didn't really care for in comparison to Sonic 3, Lava Reef, Sandopolis and Flying Battery. It's not that I dislike them but I don't really care for anything regarding those stages. While Oil Ocean Zone is a decent stage, I share the same sentiment.
There were too many bosses and I feel like the developers just did whatever and tried whatever they wanted, it looks like they had fun making all of the bosses. There are too many, (I get the Hard Boiled Heavies), but did every act really need to have a boss?
I do genuinely enjoy the Chaos Emerald stages but I can't say the same for the Blue sphere stages. I recall being able to get the Super Emeralds in Sonic 3, not that it was easy but it wasn't irritatingly setup to be a bait-and-switch at the end of the later Blue sphere stages like you said.
Sonic Mania is everything packed into a game, some things arbitrary, some things necessary, some things charming. A lot of of it is random like Angel Island Zone not being playable even though it appeared to be canonically relevant, not all of the zones having stage transitions, the old stages being blended in with the new, sometimes breaking up the flow of the game, Studiopolis going into Flying Battery is kind of a buzz kill, the momentum gained from Sonic Mania's original assets is slowed down when I have to play a stage from an older game.
Sonic Mania is incredible though.