Thanks for the post! I agree that identifying those universal maxims or norms seems impossibly difficult given the breadth of humanity's views on morality. In fact, much of post-Kantian deontological thinking can be described as an attempt to answer the very question you ask in this post. I'm also not a trained philosopher (and I lean more towards consequentialism myself), but I'll share a few notes that might help:

1. Most modern non-consequentialists have much more abstract moral/ethical theories than "follow the categorical imperative". For example, contractualists believe that these norms are only hypothetical and could only be agreed upon universally in an imagined scenario - for Rawls this was the original position. More modern contractualists like Tim Scanlon push this further. Scanlon summarizes his moral theory as: "An act is wrong if its performance under the circumstances would be disallowed by any set of principles for the general regulation of behaviour that no one could reasonably reject as a basis for informed, unforced, general agreement". The key word here is "reasonably". Scanlon pretty much wrote an entire book about what it means to reasonably reject a set of principles. The point is that these more modern deontological theories abstract away from our earthly moral disagreements. Many of them rely on a the existence of an objective moral truth that we can strive to emulate through rational discourse. In this way, it actually doesn't matter much what people or animals care about or their current moral customs.
2. Other modern deontologists are more concerned with rights and obligations. In my opinion, these moral theories are too much about what we are barred from doing (don't lie, don't steal, don't violate anyone's right to privacy, etc.) than what we should do. Regardless, given you believe in the objective existence of these rights, employing them gives a more clear-cut guide to moral action than the categorical imperative alone. These philosophers would use this reasoning to dispute your claim that deontologists are "more likely to ignore what people unlike themselves care about".
3. You write that a "deontologist might not draw any repugnant conclusions". All moral theories have shortcomings, and I think that all draw repugnant conclusions in certain hypothetical situations. In turn, philosophers who defend these theories either explain away the problem, "band-aid" their theory, or bite the bullet. There are a bunch of problems I could mention for deontology, but perhaps the most famous is the Inquiring Murderer problem. 
4. I think it's worth noting that consequentialism has its fair share of epistemic problems. Determining the well-being or preferences or desires of others, especially when they are halfway around the world, is no easy task. Plus, even if you know what would make others' lives go better, it's often difficult to know how to bring about these outcomes.

TLDR: I agree that deontology has serious epistemic problems, and in practice, deontologists might be more prone to ignoring people unlike themselves (because they are far away or because they have different views). However, much work has been done to demystify non-consequentialist theories and make them actionable - it's just highly complex. In general, I tend to agree with Derek Parfit when he argues that all moral theorists are "climbing the same mountain on different sides" in their search for moral truth.

Thanks for the post! You wrote,

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.

But in that case, the babyeater and I would argue over the specifics of the causality of violating the babyeater's maxim.

In deciding between conflicting maxims, you can:

1. contextualize each maxim's application.
2. If the contexts overlap, then elicit the universalizable consequences of the maxims. Otherwise, stop.
3. If each side agrees about the consequences, then compare the values each side uses to judge the consequences. Otherwise, stop.

In the end, I have found that either:

• the maxims applied in different contexts.
• at least one maxim is understood by the opposing side to have different consequences than the other side understands.
• the opposing sides had judged the same consequences with different values.

By comparing maxims on the basis of context, consequences, and values, you can get from some disagreement over maxims to any potential difference in values. The point is that values are not necessarily where the disagreement occurs.