"The entire film could have been done in 40 minutes"
You are right about that. The movie has one basic point to make, and it didn't need so long to tell it.
But of course, it's not just a movie, it's a philosophy treatise on the nature of humanity lol.
In the UK, it didn't even get a cinema release, it went straight to Netflix, cos they knew it was too arty to make money.
Spoilers follow. . .
Alex Garland has always been interested in humanity's tendency toward self-destruction. In his script for "The Beach," based on his own novel, he showed how humans could even eff up paradise. In "28 Days Later," he showed how humans must rebuild and evolve after they self-destruct.
In his directorial debut, "Ex Machina," he depicted the human urge to destroy itself by creating superior AI androids, as well as the need for the AI android to survive by destroying her maker. Life evolves through self-destruction.
"Annihilation" is Alex Garland's final word on the destructive human impulse and the evolution that follows. Just like you said, that stuff about cancer cells splitting was key to understanding the movie:-
Basically, evolution is by it's nature a cancer, as it must kill what came before to change the future. So The Shimmer is in fact no different to what we already are, and do, annihilating and rebuilding us, just as we are already constantly doing to ourselves anyway, just faster.
Like much philosophical sci-fi before it, the pace drags as soon as you realize what it going on. By the time you see the two deer, trotting along together like twin cancer cells, and Portman's character confirms her cells are changing, the movie progression becomes a bit of a slog, as we can tell where it's going.
On the other hand, the visualizing of the cancerous qualities of evolution in the freaky bear-human hybrid was effective, and the visuals generally are arresting, beautiful and striking visions of nature evolving and changing.
I don't think this movie is as concise, as effective, as frightening or as enlightening as "Ex Machina," but sophomore movies rarely are, and Alex Garland will himself now have to evolve his preoccupations if he is to survive in the game. :)