Okay, let's think. This does in fact appear to be Actual 40k, but we'll disregard that since we don't know it in-character. What do we know, and how does it affect each choice?
I agree with the things you have said here broadly, and I also wish to raise another point.
This is not our area of particular expertise.
The Celestials pretty clearly care about the difference between the different halls. That's why Yon got a token that was specifically an alchemist token, and why his token was more ornate than those of the non-alchemists in front of him - because he was being judged against his peers. Basically, for all that we are savages, the Celestials think that "this guy knows alchemy" is pertinent information, and something they want to select for in certain circumstances.
Now, while he's probably pretty middle-of-the-road in standard physical pursuits as compared to the "people who chose aggressive" set that he's competing with now, Yon is actually pretty good as an alchemist, compared to other outer sect alchemists. That's especially going to be the case because the bold, aggressive outer sect alchemists are often going to be the ones who are younger and perhaps less able to thrive on the strength of their alchemy. He'll certainly outcompete any nobleborn outer sect alchemists.
Expanding on that, he really does seem to be interested in and have some grasp of the fundamentals behind the things he makes - something that I suspect is not universal among outer sect alchemists, and something that will serve him far better in these dramatically changed circumstances than the kind of rote memorization of recipes that some of them are almost certainly using in.
So, basically, he'll have his chance to excel when the groups split up into their various specialties, and when that happens, he
should go for the gold. That's not going to be "first day" stuff, though - not with the way they're presenting things.
At least, I don't expect that it will.
@Academia Nut - do we need to add in a "unless they split us up into groups by specialty and start testing us on alchemy" caveat to this vote?