Note that I am leaning more heavily into addressing whether this is a plot hole than actually answering whether Eternal Mangekyō Sharingan requires transplanting Mangekyō Sharingan (though I try to answer that too).
There's a point here that I don't see anyone making, and it's this: Sasuke was supposed to have an Eternal Mangekyō Sharingan when he came into this fight. Itachi repeatedly told Sasuke he needed to have the same eyes as him if he wanted any hope of defeating him. In other words, Itachi tried to set the stage so that the conditions for Eternal Mangekyō Sharingan would be met.
We all know what Itachi's true motives were, and that he didn't actually want to take Sasuke's eyes, but let's look at this a different way. I'm going to write about the character Itachi was playing--in other words, the act he was putting on. This Itachi is power hungry and wants the Eternal Mangekyō Sharingan for the sake of increasing his own power. But not only that, he needs it because he's going blind from using his own Mangekyō. He's desperate to avoid losing his light. He tells Sasuke to kill his best friend and achieve Mangekyō Sharingan, all for his (Itachi's) own sake. And he waits, even after Sasuke gains a regular Sharingan. This is ostensibly because stealing a regular Sharingan is not good enough for Eternal Mangekyō Sharingan.
Fast forward to the battle between Sasuke and Itachi. Itachi is really on a dire strait at this point. He can barely even see. In other words, he really needs to take a chance. He needs to take that Sharingan, whether or not he still gets that Eternal Mangekyō Sharingan in the end. He still hopes that he will, of course, but he's out of time.
Now, coming out of the act once more. We might consider that Itachi had planned out this battle very carefully. There's evidence for this--afterwards, we learn that he drew out the fight specifically so he could destroy the part of Orochimaru inhabiting Sasuke. Itachi also needs to make sure his secrets are buried with him after he dies. Thus, it wouldn't be too far-fetched to say that Itachi had a script in mind. And he probably had the basics for this script planned out for years. But perhaps there's one thing he didn't account for. And that would be Sasuke not achieving Mangekyō Sharingan. So, what's he supposed to do? His script largely requires himself to be trying to take Sasuke's Sharingan. Does he just rewrite the script? There's no time to think of anything different. What would his new character motivation even be in that case, if not to achieve Eternal Mangekyō Sharingan? What he had planned out will simply have to suffice. Any contradictions can simply be explained as his desperation for replenishing the light in his eyes.
So, the question that's being asked: Does one need to take a Mangekyō Sharingan from a blood relative to achieve Eternal Mangekyō Sharingan or will a regular Sharingan suffice? There are too few cases to know for absolute certain, but there is some evidence to suggest that you do need to take a Mangekyō Sharingan. The main evidence, of course, is simply that Itachi says so (I'd like to actually find where he says that and cite it if I can. The Wiki at least seems to confirm this: "Their vision can be restored by transplanting the Mangekyō Sharingan of an Uchiha with strong blood ties — ideally a sibling — thus awakening the so-called Eternal Mangekyō Sharingan"). So Itachi contradicts himself in the fight with Sasuke? Doesn't matter, because in reality he had no intention of actually taking Sasuke's eyes to achieve Eternal Mangekyō Sharingan. It was just an act he had to see through to the end.
As for the opposite conclusion, with only two cases, at best we can say that from an in-universe and out-of-universe perspective, it is simply uncertain whether a regular Sharingan would work. But the only lore we have states that Mangekyō Sharingan is required, so we should probably lean towards that conclusion.
