This seemed like an easy thing on paper, however I'm trying to figure out the most efficient way to fill in these curve shapes, while avoiding overlaps and maintaining the "negative space." I can do this with a separate boolean pass, but I'm curious if there's a better/quicker way to achieve this.

Give the Hole SOP a shot.

Aha! Thank you that was ultimately the solution. I actually had tried using the hole SOP previously, but I couldn't figure out why it wasn't working in my case. I did some more poking around after you mentioned it, and if anyone else runs into issue with the Hole SOP, my solution was to toss a Labs Dissolve Flat Edges with the Remove Inline Points bool turned OFF before the Hole SOP:

Don't forget to delete the curves, they also remain and merge with the polygons and can greatly hinder you later.
It's easier to select the conjoint groups in the polyfill and delete everything except this group.

Also in the polyfill you can immediately choose to create a single polygon.
In fact, a regular boolean is used here.
You have 2 separate curves and one is nested inside the other. The polyfill fills each curve. And you just need to make a boolean and subtract the smaller polygon from the larger one. Which is probably what Hole does.

RGaal
Don't forget to delete the curves, they also remain and merge with the polygons and can greatly hinder you later.
It's easier to select the conjoint groups in the polyfill and delete everything except this group.

Also in the polyfill you can immediately choose to create a single polygon.
In fact, a regular boolean is used here.
You have 2 separate curves and one is nested inside the other. The polyfill fills each curve. And you just need to make a boolean and subtract the smaller polygon from the larger one. Which is probably what Hole does.

Ahh yep! I've been burned by leftover curves before too haha. I'll be cleaning em up now that I got a working solution!
In regards to the boolean method, that was my fallback manual solution, as I had a pipeline setup with that that. However, I wanted to find an alternative to booleans, as they can be a little finicky with what you feed em

Thanks all for your help!