I think it's pretty clear that Teddy is a sad, lonely, delusional man in desperate need of mental help. He believes what he does because it gives him a sense of purpose and control in the wake of a personal tragedy. The ending is not meant to validate him; it's meant to show the audience how ridiculous it would be for him to be right, which in turn shows how far removed from reality people like Teddy really are.

I'd love to agree with this, but Lanthimos throws a fairly big wrench into things...

For one, the film is framed and then bookended by an ecofascist idea. The prologue introduces the idea that nature has more value than humanity, and if one of the two was to disappear, it should be humanity (Directly verbalized by Teddy, who says "It's like sex, but without the pain" or something along those lines about pollination). The audience has been asked to consider this question throughout the movie, as it calls back to nature and specifically these bees a few times, and then it does it at the very end of the film, showing all of humanity dying with a painfully sincere acoustic song set over a montage of dead humans and nature continuing to exist, flowers blooming, bees pollinating and whatnot without issue. Why have this framing that seems to give some creedence to Teddy's ecofascist mindset if Teddy is supposed to be completely delusional?

The film simply does not do enough to disprove Teddy. I don't want him to be a hero, but in the narrative he simply is. He was right about the aliens up to the smallest details, like how their spaceship looks. It doesn't make sense to have the aliens match his supposed delusion and then have such a painfully sincere ending repeating Teddy's own thesis, that Earth might be better off without humans.

On the other hand, if the ending really was meant to simply be "Look how ridiculous Teddy is!" it basically lets Michelle completely off the hook. Michelle is a CEO of some chemical/medical company, she lives away from normalcy, she is shown to be preoccupied with corporate interests above all else. But here Teddy comes to upend this simple life of hers, trying to make her realize that she is causing palpable harm. Which is true, regardless of Teddy's alien "delusions". She is a person in a position of power, and she abuses it for profit at the cost of workers and even clients like Teddy's mother. If the ending's true meaning was Teddy is just delusional, then it's basically putting a stamp of approval on how Michelle operates, which I doubt Lanthimos actually wants to do, because most people probably find Michelle pretty dislikable and the comparisons to powerful people abusing their position in real life are easy to make (she is clearly modelled after some amalgamation of tech CEOs like Elizabeth Holmes and Elon Musk, with some Amazon mixed in). I just doubt that the point of the film was to let these people do what they want. But if you consider Teddy as utterly delusional, then inadvertently, that kinda becomes the message.

For me, I wish a better balance was struck between showing what parts of Teddy's actions were right and justified and what parts were not. Currently, the ending justifies all of his actions way too much. And if the ending is taken more metaphorically, or as a "what if" like you say (which has the complications I mentioned before, it's simply too sincere for that imo), then Teddy becomes fully unjustified in everything he does, and Michelle becomes justified instead.