Sure, but it's not true by definition that numbers are abstract.

Huh?

it seems to me that it would make much more sense to treat these number things as playing a causal or constitutive role in the makeup of our universe itself, e.g., as universals.

I don't recognize a difference between universals and abstract objects but neither plays a causal role in the make up of the universe.

Then it would no longer just be a coincidence that our world conveniently accompanies a causally dislocated Realm of correlates for our mathematical discourse.

You're taking metaphors way too literally. There is no "Realm".

The properties of quarks causally impact our quantum theorizing. In a world where there weren't quarks, we'd be less likely to have the evidence for them that we do. If that isn't true of mathematics (or, in some ways even worse, if we can't even coherently talk about 'mathless worlds'), then I don't see the parity.

It's not that complicated. We have successful theories that posit certain entities. I think believing in those theories requires believing in those entities. Some of those entities figure causally and spatio-temporally in our theories. Some don't. When you say "in a world where there weren't quarks" I have no idea what you're talking about. It appears to be some kind of possible world where the laws of physics are different. But now we're making statements of fact about abstract objects. It is very difficult to say this about mathematics since math appears likely to work the same way in all possible worlds. But that's a really strange reason to conclude mathematical objects don't exist. Numbers and quarks are both theoretically posited entities that we need to explain our world.

As far as I can tell everything you have said is just different forms of "but mathematical objects aren't causal!". I readily agree with this but since abstract objects aren't causal by definition and the entire question is about abstract objects it seems like you're begging the question.

(Edit: Not my downvote btw)