The Categorical Imperative Obscures

Hmm, unrelated, I guess. Why is it interesting to think about Kant's stab at moral philosophy? We don't bother using Newton's original notation for his laws, because we understand them a lot better by now. Is reading Kant mostly of historical interest, or is there something that can still be useful now?

The categorical imperative aims to solve the problem of what norms to pick, but goes on to try to claim universality.
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The trouble is that the categorical imperative tries to smuggle in every moral agents' values without actually doing the hard work of aggregating and deciding between them.

That would be a problem, if that is what it were doing. The categorical imperative aims to define the obligation an individual has in conducting themselves consistent with the autonomy of the will. Each individual may have a distinct moral code consistent with that obligation, and that is indeed a problem for ethics, but the categorical imperative does not attempt to help people pick specific norms to apply across multiple agents.

Kant lays out a ton of definitions and then leans on them heavily; it's classic old philosophy. Understanding what Kant said means you need to read Kant, not just his conclusions. That's a big strike against the clarity of his writing (it is a real slog to get through), but whether he achieves his intent should be judged vs. his self-professed intent, not against a misunderstanding of his intent.

The idea of a categorical imperative has never really resonated with me, and I think this description helps me understand why that is.  I don't believe ANY of my preferences or social-level beliefs are, should be, or can be, universal.  

I have pretty strong beliefs about some factors of an equilibrium that works in my extended in-group today, and I get angry at violators, and I'm happy to improve my own and others' compliance.  But I have zero claim that it's the only possible one, or the best one, or even that it's still valid in different technological or cultural contexts.  

I think the categorical imperative is a nice framework. I don't think your counterexample quite works for me. The babyeater applies the imperative, and is a morally upstanding babyeater who eats lots of its own children. Meanwhile the human applies the imperative and is a morally upstanding human who doesn't kill any humans (baby or otherwise).

Both actors are acting morally, according to the imperative. That they are not acting the same way just shows that they are different agents with different values. Conversely, a babyeater and a human could both fail to live up to this imperative (eg. the babyeater thinks that child eating is good for the wider world, and wants everyone else to eat their own children to ensure the world stays nice, but it makes an exception for itself.). For both humans and babyeaters adopting the imperative might change their policy. It changes it in different ways for the two of them because they are different.

The categorical imperative aims to solve the problem of what norms to pick, but goes on to try to claim universality. [...] The trick being attempted is to assume the thing we want to prove, namely universality, by assuming that satisfying our judgment of what’s best will lead to universality.

It sounds wrong to me to say that the categorical imperative "claims universality". I'm not sure exactly what you mean by that, but it sounds as if you mean "the categorical imperative says that if a thing is permitted (according to itself), it's universally permitted, but it also says that a thing is permitted because it's universal or universalizable, so it's circular".

I don't think the categorical imperative says that "satisfying our judgment of what’s best will lead to universality". I think it says that it does so only in so far as we act rationally (in the Kantian sense), that, in so far as people are rational, they act according to the same law, and that this law is universal precisely because it's based in practical reason (which every moral agent shares).

Suppose a Babyeater tries to apply the categorical imperative. Since they think eating babies is good, they will act in accordance with the norm of baby-eating and be happy to see others adopt their baby-eating ways.

You, a human, might object that you don’t like this so it can’t be universally true, yet a Babyeater would object that you not eating babies is an outrageous norm violation that will lead to terrible outcomes.

The Formula of Universal Law, which you quoted ("Act as if the maxims ..."), is one of three formulations Kant gave of the categorical imperative. Another, the Formula of Humanity ("treat humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means"), clearly prohibits baby-eating, no matter what norm the agent may prefer.