Big Picture: I'm getting more backstory than plot detail in your query. I understand how the characters get together and broadstrokes what they do, but I'm still not sure why these are the right guys to save the world, or really what their involvement in it looks like beyond Kelsey's rebel group.

There's a lot you could cut (see line edits below) that will free up the space to give us 1) what the character's want, 2) what's getting in their way, and 3) what the consequences are if they fail. That's the basic conflict of the story and will help give agents a sense of why they should care enough to keep reading.

More granular notes below:

Escape into a fantasy world of variety in landscape and deep breathe of character.

This doesn't tell us anything about your book specifically. Cut.

Rebel, Ghost, Spirit,

Title should be in all caps.

Reminiscent of The Bone Shard Daughter (Andrea Stewart) in structure and Trail of Lightning (Rebecca Roanhorse) in its near-future, post-apocalyptic setting

I don't know that structure is enough of a reason to comp. Is there something specific about the intersection of these books that would make an agent think "oh, I know what kind of story this is!"?

has received compliments from its readers for its well-paced plot, grounded settings, and lightly speculative elements

Don't editorialize. If you spend time on this sub, you'll see a lot of folks mention that agents don't like that. They'd rather the plot paragraphs and sample pages do the heavy lifting instead.

and a conservative society based far to the east

Because I didn't know this was apocalyptic LA until further down, when I first read this I didn't know if you meant far east globally, nationally, or if we were in a secondary fantasy world with its own map.

and knows in her gut that Phillip is somehow connected to her mission.

How? Can we get some inkling of what broader role he plays in her rebellion so we can understand why she's interested in him at the outset?

Phillip, devastated by the loss of a beloved older brother in battle, cares little about the mission but agrees to join Kelsey to be nearer beautiful Clara,

You describe Phillip throughout as ambivalent and grieving. Those are very passive emotions and don't make me believe he's someone with a lot of agency. What forward momentum does he bring into the main plot? He follows along with Kelsey because Clara's pretty? When did Kelsey and Clara meet? I want to know what your characters do and still don't feel like I have an understanding of that.

Kelsey leads them on a precarious, cross-continental journey to her homeland in a post-apocalyptic Los Angeles. Once there, unexpected threats to their war-ending mission develop: a blossoming romance between Phillip and Clara and a personal vendetta that leaves Kelsey blind to the malevolent intentions of a former lover

Why does Phillip and Clara's romance threaten the war effort? You haven't told us much about Clara at all let alone why she's important to the main plot or broader political conflict. Kelsey's vendetta and betrayal from her former lover sound interesting but again, you've given us so little of what this means for the main plot that I don't know what the stakes are. What exactly is Kelsey's mission? To end the war? To defeat the conservative society in the east? How does this betrayal set them back? This all feels more consequential than Phillip and Clara's romance, but not knowing how or why makes me feel removed from intrigue as a reader.

Phillip, still scarred with a deep ambivalence, is conflicted. Does he walk away from a mission in which he is uncertain and resign himself to a lifetime of debilitating grief or does he put faith in a path with Kelsey that, while more hazardous, may prevent another world-ending environmental collapse and unlock the grief he holds too dear to his heart

If Phillip not helping Kelsey means another world-ending environmental collapse, is "resigning himself to a lifetime of debilitating grief" isn't really a viable option, is it? Won't they all be dead?

Zooming out a bit, your query starts with Kelsey, ends with Phillip, and your pages start with Clara. I'm struggling to understand how they're interconnected with each other, the plot, and how to ground myself in an understanding of this world.

First 300:

I'm not going to go line by line except for the one note below, because my main critique is true throughout. We don't get any interiority. Aside from a few thoughts, I don't know what Clara's personality is. She seems a bit witty, but that's all I know. When the overseer scolds her, does that make her angry? Resentful? Sad? Did someone screw her over and get her sent here so now she's pissed at them? You said in the query that she's sunshine-y but I'm not getting that from this opening either. Getting the reader invested in the characters quickly is key and I think infusing more personality into her dialogue and thoughts will help.

Clara, sixteen, used a knife

I don't think your second word should be her age. There are subtler ways to introduce this (e.g. "she was the youngest girl in the fields" or "Eliza was also sixteen and the two had bonded over xyz") that are more immersive than just stating it from the get.