Kang could easily have been recast especially after all the Multiverse groundwork they had already laid. But they had made major mistakes already aside from the actor's personal issues. Quantumania was an absolute utter flop and a bad idea from the start. Kang being "defeated" by a B lister Antman in a shitty sequel that was nearly unwatchable was a horrible miscalculation in what was supposed to be the Thanos level buildup.
Pivoting to Doom makes some sense but, they left some issues in the hurried nature. The groundwork for his characterization all has to come from this movie since all we got of him previously was the Fantastic 4.
This movie has so much to carry already. You need to care about Doom, and know where he came from. Establish his threat level, connect him as a World power and honestly they need to do most of that in the first half of the movie. Then we haven't seen most of these Avengers interact since their own projects in the last 2-3 phases. All the first meeting introductions, sprinkle in the Mutants and the Foxmen version of Xmen how they connect to the new cast of Xmen has to start here somehow. So there is still clearly multiverse or time travel involved in this plot.
Yep, they had the right formula from Infinity gauntlet storyline. Instead going down the daft DC route of introducing a major bad guy without all of the stuff you mentioned.
As for Kang there are numerous iterations of him, the one antman stopped was just one. They could have used "prime Kang" (I might have made that up) in a avenger movie with a recast. I actually think someone like Denzel Washington would have been a really good Kang.
The problem with Doom, is that if he is "defeated" at the end of Secret Wars (2027). Then who's the next big bad guy for the MCU?
Quoting you "
You need to care about Doom, and know where he came from. Establish his threat level, connect him as a World power".
This is what many of these movies especially DC Comics get wrong. For Doom specifically there is no build up. They could have put in a end credit scene (in brand new day) with a badly beaten up (off screen) avenger on the verge of death. (Ideally they would kill said avenger but that won't happen). All the avenger would have to say "...Doom is coming..." Or something like that and fade to black.
That would create anticipation, especially for example, if the screen faded to black and all we heard was a flatline sound of no heartbeat of the said avenger...
The implicit message is there for the audience. Doom is coming and he's killed an avenger already. Even thanos didn't do that (the snap not withstanding).
Instead we get marvel, bringing back Captain America and RDJ, which smacks of desperation from movie executives who should know better.