IMO, Thoughts on the series as a whole:
I've never liked the entire opening of the series, it was played way too safe and boring as a result, also nonsensical. The opening chapter with Giyu/Tanjiro is pretty solid, I can't deny that. But past that premise it just starts to slip. Back then most people thought it was boring, stale artwork, and going nowhere. And I still think that holds up. It wasn't building into a longer narrative and was at the bottom of WSJ constantly, most readers were assuming it would be axed.
So let's break it down:
The random Demon attacking the village proved to Urokodaki that Tanjiro was too soft, it seems nonsensical for him to accept a timebomb like Tanjiro for no real reason. A little backstory about WHY he would do it... might have been better. He would have MORE reasoning to reject Tanjiro as his previous disciples proved too weak for the task at hand. I just feel like this could have been removed entirely. The training arc overall wasn't exactly special, mostly because training arcs have been done to death at this point.
The Demon Hunter Selection makes the least sense out of anything. They dump them on a mountain where there are just randy demons who happen to exist there. And they need to survive. Who in the flying fuck even thought this up. Every time I think about the section I get mad. I get they have the demon that killed the previous disciples which relates to Giyu. But WHY. Why was the demon there and why did no one know about it? Don't you think plenty of aspiring Hunters would have died? Wouldn't that have been a tip off? How is this method even good for selection? Couldn't someone just hide until dawn? etc etc etc. Logically it makes no sense. What does make sense? To become a Hunter you have to train under a Pillar (or Former), who know just how powerful demons can be, the Master then sends the Disciple to complete a mission and THAT is what qualifies you as a Demon Hunter. If the Master fakes the results, the Disciple would eventually pay for it. This also puts the Masters' at risk since their reputation is on the line to get the job done properly without losing a Disciple.
Following off that, the Muzan reveal was weak. I get the Author quickly wanted to introduce the main villain but should've done so WAAAAY fucking further back. When Muzan was first revealed it seemed to everyone that he WASN'T the main villain, until we were told otherwise. That's how weak of a presence he gave off. The Tamayo arc beyond that is decent, and the following content up to the Pillar Meeting is okay. BUT the Author dropped the ball BIGTIME by introducing Inosuke. The error I see is that Inosuke didn't have too solid of a character arc and ate up a lot of screentime, screentime that could have gone better off to other less developed characters such as Genya and Kanao. Hell even someone like Zenitsu needed more screentime.
The weakest part of the story is the step between the Pillar Meeting and the Train.
We should have had an arc where the Lower Demon Moons all indiscriminately attack the Pillars, which would give up a little more information on everyone and build them up as characters. Their solid defense against the Lower Demon Moon forces would hype everyone up "We can win!". We can have Tanjiro watch Rengoku and build up their relationship BEFORE the Train arc so when Upper Demon Moon 3 pops his head in, we think Rengoku stands a chance before the shit hits the fan. It would also feel less out of nowhere. The motivation for the Lower Demon assault would be for Muzan to distract the Demon Hunter forces for some goal, rather than him just off'ing the group blandly.
Following that the story is good through the final arc (I'm sorta iffy on the hidden-sword making village arc but whatever).
Because we never got enough information on Genya, Kanao, Muzan, most of the Pillars, Zenitsu, Inosuke, Yoriichi, and more, it's fucking FLASHBACK CENTRAL TIME.
Genya just wasn't developed enough as a character for me to care about him, nor was Kanao. It's almost like we should've had them join the crew way earlier to get some development going.
Zenitsu's duel against his disciple brother was meh, the letter wasn't enough information to foreshadow anything, everything came out of nowhere.
Inosuke's arc was all over the place and wandering. I don't really understand why he needed to be included at all, he didn't really reinforce Kanao and both characters felt underdeveloped. I think cutting Inosuke and giving more screentime to other characters, giving out some of his characteristics to other people, would have just been better. I get he's like one of THE mascots for this series, so I am reluctant to think that way, but it's a bitter truth. Again, despite being around Inosuke almost the entire series (like Zenitsu), we still needed a flashback to explain their character.
I feel that in a FINAL arc, you shouldn't need to be explaining so much shit. Especially not explaining the fucking characters. Like goddamn.
The ending to the Muzan fight.
Muzan needed to just die. It somehow felt unsatisfying and frustrating. I'd like to mention the whole Drug meme right off the bat. Having the billions of fucking drugs that Tamayo and Shinobu dosed him with, was just annoying. I was fine with the initial drug. Okay. But the two further ones? It felt too much like, "And ONE more thing!", every other chapter. In fact I was sitting here thinking that there was the 5Head strategy of Muzan finding Nezuko who comes to help, devouring her, only to realize that Tamayo predicted that he MIGHT do just that and it backfires turning Muzan into a human. Muzan screaming "I conquered the sun" while having returned to being human would've been hilarious. Then for him to rapidly age to death. Instead?
Muzan injects Tanjiro with blood and he manages to overcome it due to the power of drugs. Like I'm so confused, what even was that.
I would've preferred about ANY other type of ending instead of doing this fucking 3 chapter waste of time bait.
I also can't stand that in the final chapter we didn't get to see Tanjiro's back, resting while he looks up at the night sky. Someone calls out his name, leaving us to wonder his fate due to the Mark.
I mean I enjoy seeing all the kids running around but like others have said, it seems more like a bonus chapter after the final chapter. I wish we had just gotten ONE more with Tanjiro. My only other thought is that the Upper Demon Moon 1 fight was, and still is goddamn amazing. Despite all my grievances about everything else, that fight was just enjoyable, mostly how it displayed the sheer desperation to win.