I think we should remember that Orpheus is a gith and cruel. I personally prefer his route to Empy, but both are not acting out of the goodness of their heart.
And I would find it interesting to attack him, followed of course by an instant game over, since Orpheus power was the only thing, stopping the brain from making the group part of the Grand Design again.
I wouldn't say Orpheus is cruel. Harshness of the gith yes, definitely, as well as the fact that what is truly important to the gith is the well-being of their people and the fates of other races and the inhabitants of Faerûn are unimportant. As long as a member of an alien race does not become an ally of the Gith, they have high notions of honor and the Gith know how to be grateful. Orpheus, moreover, is a wise and skillful diplomat (he builds ties with the Githzerai, as we also learn in the epilogue from Lae'zel, who becomes a diplomatic representative, and given the history of the Githyanki and the Githzerai, that's saying a lot). We've already helped Orpheus, we've already become allies, and if he just forcibly changes someone who has become an ally to him without asking, that's not very honorable for him. It makes much more sense to offer and find a volunteer, especially since someone should understand the meaning and necessity of the act. As a diplomat, Orpheus also has to understand the mindset of other races as well; the one he's trying to forcibly transform may have someone who loves them. Even if they don't attack him at the moment (although I would choose this particular game-over, if Orpheus started changing Astarion, my character would think of nothing else at that moment and I would never free Orpheus again, and would play through the game exclusively in alliance with the Emperor, who definitely wouldn't do that, he's an Illithid himself), after defeating Brain, there's nothing stopping Tav and whoever he made Illithid from getting revenge on Orpheus for that and killing him, and that would bring harm to the entire Gith people and their fight against Vlaakith. Yes and the image of Orpheus it ruins - this character is interesting, well shown (as well as in general githyanki, Lae'zel and their culture is revealed interesting and worthy), causes sympathy, and the bastard who just takes and turns into illithid your companion - well, one bad playthrough, and after that just never want to free him, no matter whether tadpoles were used or not, which also spoils the storyline of Lae'zel and githyanki.
My point was, that there should be stakes and consequences to use the tadpoles - either for Tav/Durge or a companion using the tadpoles. As of now, you only have a consequence, when you choose a specific evil path and botch a dice roll ( though a very high one). I think the tadpoles should be a high risk and high reward thing in general. You should be less able to resist the more tadpoles you consume, because you become more illithid the more you take in and use the power. Minthara is pretty open minded about it in general and doesn't mind the physical changes at all when she consumed the astral tadpole.
There are consequences for Tav/DU - I had as high a roll as ldo58 for not becoming half-illithid (even I think it was 30 the first time, but I could be lying, I don't remember the exact number) because Astarion and I took almost all the tadpoles for two, when I shared with Minthara in the next playthrough, the roll was still quite high. In Honor mode, I don't use tadpoles myself because of this. I know Minthara is fine with the physical changes, but she won't want revenge, she'll want to become an Absolute and take the power herself and rule.
And of course a character changed by Orpheus or Empy is not a zombie, but they probably know, that they risk getting controlled by the brain, if they kill the only person, who has the power to prevent that. And even though, not all companions are as smart as Gale or as knowledgeable about illithids as Lae'zel, but by that time, they know the dangers and the full story of the protection.
Not at this moment, when they risk falling under Brain's control, but later, when they use the Crown of Karsus themselves. When Karlach becomes an Illithid, in a fight with Brain, she must subdue Brain using the Crown of Karsus. There is a moment of choice when Tav can make the evil choice to attack Karlach and take control of the Brain. But Karlach could do it herself and subdue both Brain and Tav and everyone else, she doesn't do it not because she can't, but because she never wants to do it. Astarion and Minthara will do it. In Astarion's case, Orpheus will be very, very hurt afterward, and Minthara will probably be satisfied with that outcome, and she won't let go of this power that has fallen into her hands on its own.
Illithids are DnD response to "ship of Theneus" (google it, its fascinating thought experiment).

Illithids copy your whole life, personality, experiences and thoughts into its brain, and while it may seem like that person is still there since that is all they contain in moment of transformation ... the original person died in the process and this is entirely new being ... ergo
(we see this in the example of Karlach, who changed a bit, but retained many aspects of her personality)
And it will dissapear bit by bit as the time goes ...
SInce the same "copy all" process will go with every single feeding.
Yes, thank you! Also, the illithid has a completely different set of hormones, these hormones are clearly different from the original state and, under the influence of these hormones there are quite serious changes, + every time we eat the brain we get a copy of the personality. Illithid is no longer a warm-blooded mammal, it's a different organism altogether. Karlach says her engine has cooled down (illithids are clearly cold-blooded, more like an amphibian, like a fish). Over time, enriched by these experiences from multiple meals, and naturally a different hormonal background of the illithid, the personality will change greatly, gaining knowledge and experience of the surrounding world of all these eaten humanoids. Karlach becomes much calmer after the transformation, her previous emotionality disappears literally immediately, which also shows the changes in the hormonal background and neural connections of the illithid. Illithid gets the life experience of the eaten humanoid + new hormonal background of illithid = in time there will be little left of the former worldview of the individual (illithid will also stop humanizing food, but will distinguish who is a friend and who is an enemy). But, unlike the real world, there is a soul physically on Faerûn. The soul is still there, and it is this soul that Ansur tried to pull out of Balduran. In the Emperor's story, the Emperor's body itself lay sleeping, and Ansur was talking to Balduran's soul. But as the Emperor was under the influence of a new hormonal cocktail, he liked his new form and had no intention of terminating it. Illithids tend to like a new form, even if the character had previously perceived the transformation as something immeasurably horrible, new hormones affect the personality. Even Orpheus, who asks to be finished off so he doesn't “live as an abomination”, can be talked out of it and will live. Ansur wanted to free Balduran from his new shell, he wanted to return his soul and allow his soul to enter the Domein (he probably expected to fish Balduran's soul out of the Astral Sea and bring it to the Fields of the Dead, since the gods themselves have no power over Illithid souls, this is an assumption since Ansur considered this act a mercy).
If viewed from the outside, from the eyes of someone who knew this individual before, then, under the influence of the new hormonal cocktail and the experience of the meals, that viewer might say that the illithid has lost their soul, themselves, everything. Neuroscience - hormones guide our decisions.
I liked how Larian showed the interaction between Ansur and Balduran. Also, as Karlach is still there. Will she change by eating other people's brains - yes, she will.
As the matter of fact, our protagonist should have get game over, once we decide to turn.

But that would be boring.
Yeah, and this is basically impossible to display in a game (for the player). Imagining how we'd feel and what we'd think if our human hormones were replaced with illithid hormones is impossible. Not to mention the experience of being fed. It can be reasoned that a newborn illithid that has not yet eaten another's brains is still guided to the greatest extent by the experience and worldview of their former personality.
Yes, there might be players who want to save the realm before all, but I feel most take the neutral stance of wanting a good ending for themselves and their pixel friends.
That's right. Otherwise, there's just no point in all this saving the realm if you and your friends aren't to live in it.