I see it the other way around. It is using tadpoles that need to have more consequences. You have the positive ones (powers), but there are barely negative consequences to balance it out. From my perspective, the more you use the powers, the more you should want to use them. So past certain tadpole consumption/power use, it should start by changing some dialogues so you can see regular answers but only the "Authority" answer is available to use unless you pass a wisdom save. As in, maybe you have great persuasion and you could have used that, but instead, your proto-illithid brain resorts to power use because that is what an illithid does. And past another point, normal humanoid feelings should start disappearing. Romance makes no sense for illithids unless is to manipulate as the emperor does. And past a certain point, if you have consumed the astral tadpole and many tadpoles, you should crave more tadpoles, illithid brains, and, well, regular brains. Because you are transforming. This means that by the time the emperor tells you that you should turn into an illithid, your character might actually "want" to transform. And resisting that want should be really difficult or even impossible.
I like that a lot. This is beautifully mirroring the Dark Urge dynamic, where you can stay who you are (a murderous puppet of bhaal) or emancipate yourself from that evil influence and find your own way (afaik, haven't played Durge yet), while with the tadpoles you can choose to either stay on track of your own personality and values or give in to the corruption of power and lose your identity / soul.
Like stated many times in this thread before, as of now you can use the tadpole powers because you're power hungry and want to dominate or because you sacrifice yourself as a "true hero" in order to overcome the "netherbrain". It doesn't make a difference in the end: no corruption, no sacrifice. Or as Nicottia perfectly put it:
It's all pathos and no logos.
