The MCU worked because you had a collection of movies that each could stand on their own reasonably well and a decent way to tie them together.
Ironman, Thor, Hulk, and Cap all got their own movies (two of them in the case of Iron Man) with the SHIELD people tied in relatively well as supporting stars in the various other movies.
Personally, I would have done a Widow & Barton focused movie in there as well but whatever.
Then you had Avengers and it had three relatively strong leads story wise in Tony, Steve, and Thor. The story was relatively simple, the acting and CGI was well done, and the universe worked relatively well.
Then you had the problems start. Thor 2 just doesn't really make sense in a lot of ways, I mean you have aliens attacking London and yet Thor is the only one responding. It was also basically totally ignored by everyone else in the MCU.
You had Iron Man 3 which could work just fine on its own but is, again, largely ignored and doesn't really pass the smell test when it comes to the broader MCU. The President was kidnapped, the Vice President was cutting deals with terrorists, some tiny ass corporation was creating Cap/Cap+ tier super soldiers enmass, Tony Stark was presumed assassinated and the CEO of SI kidnapped at the same time. And yet no appearances from any of the rest of the crowd and it was all basically ignored by the broader universe afterwards.
Then you had Winter Soldier. Again, on its own its a mostly decent movie. Except it was the vehicle used to progress the meta plot and the movie in this phase with (far and away) the most relevance going forward. And in the broader context of the MCU, it was a fucking stupid movie and plot. The easiest solution would have been to take WS and end it with the presumed death of Fury and Cap & Widow on the run. Have Avengers 2 start immediately afterword's with Cap & Widow gathering the Avengers and them as a collective whole dealing with Hydra and the takeover of SHIELD. Enter Wanda and Pietro as part of Hydra's "Avengers" team that has Bucky at the head.
Instead we had WS into Age of Ultron and with it the abortion of the MCU. AoU worked relatively well on its own, but it led basically straight into Civil War.
Civil War as the last movie of that Phase (after Strange and Black Panther, and maybe concurrent with Thor 3) might have worked. But instead you basically go AoU to CW and CW royally fucks things up. Wanda is a Hydra terrorist who should have been executed for her actions, instead she was thrown into the Avengers. Cap suddenly decided that all oversight or caring about the little people was beneath him and only his friends matter. Black Panther is thrown in to set up his own movie and yet in the process makes him a fucking idiot who should become an international pariah instead of any kind of leader. Vision just doesn't get the screen time really needed to set him up properly. And Tony is forced into the role of bad guy despite being the reasonable, sane, adult in the room.
Black Panther and Strange were basically stand alone movies with minimal to no connection to the broader MCU ecosystem. Spiderman was also brought in with his own movie, but there were no Hulk, Ironman, Vision, Widow, Wanda, or Hawkeye movies.
Simultaneously, the underlying MCU meta verse was drastically expanded. You had Strange bringing in magic in a big way and you had Ragnarok suddenly taking the Asgardians from advanced aliens into actual gods (despite denying that in Thor 1). And you had Guardians of the Galaxy bringing in Ego.
All of it flows into Infinity Wars/Endgame which kinda just collapses under the weight of an unstable foundation.
And IW/Endgame were handled really fucking badly in terms of impact on the setting. Half the worlds population vanished at the end of IW. Realistically, another 25% probably died as a direct consequence of that. Nations don't survive that kind of thing. And then five years later all those billions of people Snapped away suddenly pop back up. At least half of them are going to die in the immediate aftermath of that simply because the infrastructure will no longer exist to support them.
Food production post Snap is going to drop drastically because only half as many mouths have to be fed. It takes an entire growing season (absolute best case, assuming that all of the needed infrastructure is already in place) to substantially increase food production because it takes that long to grow the food in the first place. People starve in less than that growing season. You suddenly threw ~3.5 billion more mouths to feed into the world with no warning or preparation. That is another event that, in addition to killing a billion plus people, will collapse governments the world over.
Basically,
Phase 1: Great
Phase 2: Pretty solid in a lot of respects but on a downward trend
Phase 3: What the fucking fuck?
Phase 4+: Meh, really not worth watching as anything but individual stand alone movies that might or might not be any good.
If I could change things, I would have reordered Phase 3 in a big way. Black Panther occurs before Civil War, and probably tie in Barton and/or Widow or (maybe) Vision. Strange occurs pre Civil War and tie in Wanda, with a fair bit of character growth to overcome the mind controlling terrorist problem. Thor 3 is, again, off Earth and probably has Hulk or Vision and I would tie it in with the Guardians of the Galaxy focused more fully on Thanos.
Tie together Spiderman, Ant-Man, and IM 4 into one or two movies as a kinda minnie arc. Take the opportunity to show more of the politics of Enhanced and start really setting up for Civil War while also getting people more established for Thanos.
If Civil War must occur then rewrite it so that Cap has an actually reasonable position and is less of a selfish terrorist who is an insult to the word Hero. Or play fully into that theme and don't try and make Cap into the good guy, instead have it be his fall from grace with him realizing how he has fucked up when he has Tony on the ground and is about to kill him with his shield and Cap and Bucky go off to find themselves.
Infinity War is basically fine as it is. Thanos wrecks Asgard and attacks Earth basically a day or two after the warning of the attack arrives. The Snap happens, the team tracks down and kills Thanos.
Then you have a few movies dealing with all of this. Put Iron Man 4 in here and have it be about Tony Stark seeing no choice but to really embrace his power as he works to hold the world together. Governments have collapsed and someone needs to hold civilization together in the face of billions dead and sheer chaos. Have IM4 be about dealing with the high level consequences of the Snap and Tony's grief over it all (and the death of Peter). At the same time, have a Cap focused movie dealing much more with the ground level consequences of the Snap and dealing with the warlords and quasi warlords using their powers to try and make themselves the local power.
Then comes Endgame. Show them actually preparing for the return (and maybe staggering it, so that say only 10% of the people per year, randomly selected, reappear and they do so next to friends/family/etc.).
Post Endgame movies deal with a very different world. Maybe take the opportunity to add Mutants to the MCU, make it a side effect of the Snap.