Scaled to size, this is eye-bleedingly small print with massive wall-of-text style paragraph blocks. Not in love with that at all. At least you kept it black print on a white background. At double page size, it still feels small. Not at all ideal. I mean, I know you're trying to fit an entire planetoid into 4 pages. Shoot for the moon I suppose (or planetoid) but I think the ambition ended up causing some issues here.
Okay, so into the meat of the text. First section is backstory and setup. You lead with the inciting incident and then jump back in time to how she was changed. This is jarring since I only know one sentence about her and already I am now having a flashback. Most of these are fairly long sentences too. I'd restructure this as well as try to find ways to break up the sentences. One long, one short, one medium, another short, etc. To help illustrate how jarring this is, here's the number of words per sentence in this section: 22-31-27 / 19-17 / 50.
Not a single sentence under 15 words. Most of the sentences in the first paragraph are similar in length so they feel monotone in addition to being run-ons. And the entire last paragraph of the first section is a single long running sentence. I feel qualified to admonish you on this since I have to admonish myself as well. I suffer it as well. If left to my own devices I will write a paragraph-sentence. I seem to subconsciously expect a few commas to do all the heavy lifting. Break it up. You'll thank me for it later.
This trend continues through the whole document from what I am seeing, so I won't keep harping on it. Just know that it is harming your readability a lot. I also wish you'd made use of bold or underline at least as well. Italics do call out some things, but when it's this much solid text, everything kind of feels 'samey'. The dragon names are an example. Being bold would help break up the blocky feel and draw the eye to them. I do like the explanation about why the planet hasn't fully crashed. It was something I wondered. It was immediately answered. Always a good thing.
Notes to the GM work by and large. You make it clear this isn't about providing stats, but offering a framework setting to do what you want with. I approve. With only 4 pages, I was wondering how you intended to fit a whole planet in here along with adventure and stats. Instead of trying to cram things even smaller, you opted to focus on one element. The world. Some people may knock you for this, but I am alright with it. The down side is that if you're only going to do one thing, do it well. It's going to have to really pull a lot of weight. Especially since gameability is going to be limited. That said, I could probably have figured out usage myself. A sentence or two to clarify the things I just mentioned and I can work out what to do with it depending on my group dynamics.
Setting notes are clear enough. Since there's no statistical information, you try to convey what happens here and how things would behave. It should let a GM plug this into their system with only a little side effort. A few house rules. Ranged is limited to X, voices don't carry past one facet, etc. So far, nothing to raise an eyebrow over. Side note: Smart hyperlinking the webpage.
And then I get to... um.. wait... the story. Okay, so you put the backstory and GM information front-loaded and the Player facing elements at the end. Wouldn't have been my choice, but hey. The reason I say that is because if I go to yank this out after sitting in a folder for a few years I start with a skim. Refresh myself. “Oh yeah, the thing with the witch and the dragons.” And I might start to tell players something I shouldn't because I didn't skip ahead before suggesting it might be a fun thing to plug into the game. Granted, I don't tend to ask my players. I just do things. Other GMs I know do though. So I would put the 'what is going on as the players know it' section first.
Page 2 breaks up the map key in a weird way. You start with specific facets, then mention flora/fauna, then go back to map elements then to water. For me, it would have been more logical to start us with the note on flora and fauna, then mention water, then lay out the general map overview, then down to the specific areas. This flows in a path of things I personally like better. Flora and fauna are a planet-wide thing. Same with water, so they come first. Then when we get an overview, we see how it is all laid out, then specific elements of interest are pointed out.
210ft interior isn't small, but I don't know if my concept of 'extensive tunnels' and yours align. At least not if there's any chambers larger than a bedroom. It's magic and fantasy, I get it. I won't go whining about earth-like gravity making a swiss-cheese cave planet unstable. None of that matters. Hand-wavey works. God help me trying to work out tunnels in a sphere of gravity for something only 210ft thick from top to bottom. I don't know. I don't hate it, but it does feel like homework. I'm already going to be doing a huge amount of legwork on this one, so that feels like something I'd probably just shrug off and not even bother mapping. Just a system of rolls to not get lost and random chance to find some chamber somewhere down there with something in it.
So now I sit here trying to decide how I feel overall. I nitpick. I always do because that helps you find what you want to change and what you disagree with and want to maintain. I think if I were trying something this ambitious, I would have reduced the backstory some. I probably would have just noted what used to keep the planet up and that something had interrupted that. I'd have probably dropped the dragons entirely. I like them, but I think I'd have rather focused on the 'essence' part. In this case, that seems to be the water. Having something happening to the water that is the essence means that is the real focus of any story built here. Putting up multiple hooks and making the planetoid into a sort of mystery would have been absolutely amazing.
I don't know. Just random thoughts. I also say that if you plan to revisit this, work on the blocky nature of the text. Use call-out boxes for things, bold some words, underline or color something here or there. And above all, break up the sentences into smaller blocks. I often say shoot for the moon on these jams and you certainly did. You pulled the whole damned moon down for us to explore! It's doing things no one else had the brass to try. Did it work? Depends on who you ask I imagine. For me, it was a solid framework, but a rough read. A good starting point, but one I would likely make major changes to before bringing it to my table. Without much in the way of game-stats, tables, or even location maps (like the tunnels or hut), I am doing 80% of the work to make this into something players can explore and play with. I could absolutely see this being expanded into something much grander. If you do, I say pick a system and go with it. Maybe add a page to note conversions into other systems if you wanted. Good job.