To me, the central issue with the movie was not the slop CGI, or even the dialogue. It was that Scott being the protagonist of the movie felt a bit...random. Like Marvel Studios had a dartboard, and decided that they needed another Ant-Man movie and a Kang introduction, so they just put them in the same movie.
That's just the sort of thing that happens in ongoing franchises sometimes, when the timings of things align in certain ways. It's no weirder than Darkseid and the New Gods making their debut in Superman's Pal Jimmy Olsen.
The best villains are a dark mirror of some aspect of the hero, riffing off of them in some aspect thematically, or else have personal history with the hero. Kang has essentially no relation to Scott except he happened to be the Avenger that showed up.
But that's why it was relevant to Scott's character arc. The story was about Scott struggling with his reputation as a major hero and questioning whether he was worthy of it, so pitting him against a villain who considered him inconsequential was the way to illustrate that.
Also, people tend to overlook that the film wasn't called Scott Lang, it was called Ant-Man and the Wasp -- and there are two people each that use those championyms. The movie was as much about Janet and Hank as it was about Scott, and Kang was critical to Janet and Hank's story because he was the reason Janet stayed lost in the Quantum Realm for so long, a threat so great that she chose to strand herself there in order to keep him contained.
Adding to this, Scott had no character arc to speak of in the movie.
Because, again, it wasn't exclusively his movie. He had two whole movies centered on him. The advantage of a series is that you can choose different focuses in different installments. This was Hank and Janet's turn in the spotlight.
What this is all very similar to is Captain America: Brave New World. There is no reasonable argument why Sam Wilson should have had the starring role in an Incredible Hulk sequel. Okay, I realize Universal may have been upset if Bruce Banner was in here, but...maybe don't make a Hulk sequel then?
Well, yeah, it was a sequel to TIH, but it was a sequel that focused on Thaddeus Ross, and Ross was a member of the government, so it made sense that a story about him becoming Red Hulk would involve Captain America.
I don't find much similarity between the two films. Quantumania was about introducing new characters and elements into the MCU. The flaw with CA:BNW is that it felt too much like it was just tying up various loose ends from earlier movies.