So starting with the cover, because the cover has a whole page dedicated to it. “It's a bold strategy Cotton, let's see if it works out for him”. Including a cover at all eats a quarter of the space you have to work with. A fair number of us did make this choice, but with only a few exceptions it was used to show a bunch of extra information through various means. You instead chose to let the image do the talking and told a story with that artwork. The only thing here other than the title is that it's Appendix N Jam and your name. It does pull weight though. The massive Ifrit dominates your initial view. Your eyes then recognize the smaller figures in the foreground with their two very different personalities. Finally you see the figure in red who almost blends into the surroundings as if he belongs there even more than the dominating figure before him. It all works very well to spark curiosity and represent the sense of many old pulp novel covers. The only element that is at all out of place is the Ifrit's design feels more stylized than the other elements of the image. Not a shock since I assume it is photo bashed. There's probably no fix for this and it isn't so jarringly different that it detracts from the art.
Art focus carries over to what is clearly meant as the back cover. It again leans into the book cover aesthetic which I love. If I was going to nitpick, I wouldn't have the three suns lined up in a neat row like that. It feels unnatural. Also, if I print this out, the back cover is going to be on the front facing side of the page and the front cover is going to be on the rear facing side of the page. The title repeats. I say use this as a call-out of theme instead. Many books put some bold phrase here to set the tone of the words that come after.
Level is clear, game is clear (we all know 5E is 5E D&D). Short and sweet italic text sets a tone and isn't intrusively long. Immediately after is a clear direction of where the players are aimed and what is going to be inside. Good. There's instant setting here with the sun mechanic. Clear ways to avoid constant fire damage, but also a clear limitation on flying. Smart players can find a way around this, but probably it means burning off valuable spells. There's an immediate way out of the sun and a clear dumb choice. Players can manage a very short wait and rush to the shelter of the canyon wall to progress further. Good. Bold unthinking folks might burn to a cinder, but most players are going to be cautious enough to figure it out even if they don't make the roll when it gets suddenly dimmed out.
Encounters are very well balanced in nature, not just fights. 4 is pretty fun since players are probably thinking they die if they lose, but avoid the fight simply by not winning. Having traders on 5 who can whittle away their valuables is smart too. 6 creating an ally if they aren't murder hobos is pretty solid too. I like it.
Into the following pages, the art continues which is nice. Rough for the printer, but undeniably pretty. I can skip the first page to save ink, but there's no way around it with the others. I didn't download all of the variations you had since I wanted to focus on the core voting element here, but if you haven't already, consider uploading a low-impact version that's going to lighten the ink load. Maybe faded or heavily simplified in a gray scale for those who just want to print it out even if it means missing out on the lovely visuals.
So far, I had recognized most of the bold entries for different entities. I have no idea about Isatum-nisu though. Not sure if I just don't remember them or if this is unique to your module. If they are pre-existing in the bestiary, great, but I find myself looking for a stat block somewhere for their default. I often get around this issue by including a source and page number in my text alongside the name of a creature. (Think the old White Wolf books. WW15 pg. 221) This also applies to loot so I can instantly pull it out in the middle of a game without having to stop everyone to answer what the item is. One thing about these short modules is that I tend to use them as 'quick draw' items in a folder. That means I usually haven't spent hours pouring over them right before the game and bookmarking pages.
Map side note: Fallen star as a spaceship appearing on the planes of fire implies this isn't a plane so much as another planet the portal took them to. It's an interesting idea, offers unusual items some games don't always normally have, and totally falls into the Weird Science Fantasy element of the Appendix N.
Second map thoughts are in a different vein. Everything about this place is fire. It's obvious right? But this is a location where they are protecting it from others who also live on a plane of fire. Almost everything and everyone here is fire resistant. How much of a shock would it be and an 'aha' moment if players pull the lever and are suddenly blasted by freezing cold instead?! Just a thought. I would probably alter this if I was running it.
Single solution puzzles normally bother me, but unless the players are in a constant act of healing up, it seems likely at least one of them has burned skin at the moment. It might do well to drop another hint earlier in the setting by someone lightly burning their hand to open a vault door or something like that. Just a thought.
Everything comes full circle in the end. Not sure how I feel about the abrupt end being a default combat after witnessing that what they came for was BS, but could be worse. It's meant to be read out, which I never actually do. I tend to just get an idea of what is going on and RP the event as I see fit. Again though, it's not a giant wall of text to follow and I can just as easily get the idea of things from reading it as I would from a more bullet pointed situation. No fault here, personal style. Since it's not super long, I am not concerned.
You found a lot of good ways to pull theme, visuals, and writing into this. Gameability seems solid too. Even though I don't run 5E, I can probably run this without much effort and just convert it to whatever system I want. There were a few hiccups like the confusion I felt about the natives, but I could also just say they are human variants in terms of stats without batting an eye. I did feel like the map was a bit linear. A long path with a few side options you would need to backtrack from to reach some bonus content. This remained true on the volcano. If I was going to change only one thing, I think it would be to rework the canyons to offer you multiple paths. Things where picking one over another means missing things, but trying to loop around to everything means more risks of encounters and sun issues. Fundamentally, they're trying to aim for a singular location so they're probably going to keep going in a rough line anyway, but giving them options is always nice. It prevents players from feeling like they /have/ to go a direction just to progress the story. Nice job on this.