SAF - long time ago but could have been sacked on several occasions in the 7 years it took him to win the PL.
Moyes - Obvious disaster from the get go. Clearly well out of his depth.
Van Gaal - After a rough start, we actually started looking like a decent team by the end of his first season. Second season we retained the new ability to control games and dominate possention but lost any attacking flair we had so ended up toothless. Still controlled games though, even if it bored everyone to death.
Mourinho - Undid anything positive re possession based football Van Gaal implemented because he doesn't believe in systems or style of play, and instead went ultra pragmatic. Won a couple of trophies doing it but set us back a decade in terms of playing style. More importantly, he introduced the toxic mentally we're still suffering from today. Picking players based on patronage and who he decided to not throw under the bus when it didn't work out for him.
Ole - Wanted and tried to get us playing swashbuckling football of old. Worked for a few games but quickly became evident that it relied entirely on momentum, and once that stopped the players didn't have the self believe to regain it.
Rangnick - hired to try to implement a high pressing system that the next coach could build on. Players went with it for a couple of games then downed tools forcing Rangnick to revert to something in the players comfort zone. Which still didn't work because the players already made the call he wasn't worth their time listening to since he'd be gone end of the season.
Ten Hag - Also tried to implement a style early on. Even with a full pre-season it was an even worse disaster than Amorim is currently facing. So he gave up and reverted to the former system the players are used to, because they just can't adapt.
TLDR - recent managers have all gone to shit after quickly abandoning their preferred systems to cater for the squad they inherited. Once you do that you give in to the players and accept their preferred system is the only system that works. You need to win a lot of silverware and wait for senior players to move on before you have any real authority to implement your own tactics. feck that.