Hopefully Ravirex here will get to eat the Ysoki Kineticist.
Edit: THE FUCKING INVENTOR GOT 3 FATAL D12 CRITS IN A FUCKING ROW
Thinking of updating Ravirex to a mirage dragon, hoping to mess a bit with an old gm that will play.
Fuuuuck, I had a similar idea but the ones I looked at didn't really fit, Mirage would be the perfect replacement. Go for it.
generally spoilery
In general, I've not found it as lethal as many report, but my players started with no PF2e knowledge and one had no ttrpg experience at all, so I structured the floors around introducing them to new mechanics, techniques, tactics, abilities etc.
Mr Beak and the shrine Corpselights on the ground floor are the biggest threats starting out - they can easily down a level 1 character on a crit and the Corpselights could potentially wipe the party in one with their beam attack. Killing a party member here did have the advantage of letting me run a long ass funeral for the dead character then bringing him back as a zombie in the deadtide event the next night though, so while the fights aren't necessarily fair they can help in establishing tone and characters personal motivations.
Middle floors will largely depend on how reckless your players are for kill count, but they should be familiar with the mechanics and be able to handle everything if they're careful and not afraid to flee if everything goes south. Middle floors suffer the most from the single-PL+3-enemy-in-a-small-room problem, but we found each fight generally brought something unique to the table and kept things interesting.
Last few floors are level dependent - if you're doing XP rather than milestone the party can easily get a level ahead of where they're intended to be by scraping each floor for loose experience like mine did, but it can just as easily leave them a level behind if they're regularly missing secret doors or ignoring sections of floors. If they're ahead of the curve the bottom floors have danger around every corner, keeping the party on their toes even in fights they're fully prepared for. If they're on milestone, fights like Ravirex will kill characters even with rolls as bad as mine. If they're behind Belcorra will TPK them as soon as they get there. There's less roleplay and engagement with Otari, but it has the most engaging and dynamic combat, and replaces the roleplay by moving the story down into the dungeon. Most of the actual story for the first 2 books is about Otari rather than the Gauntlight, but the third lets them actually engage with the BBEG and shutting down the Gauntlight, so it doesn't need the extra stuff from outside to keep the players interest.
Generally you can run the whole thing straight from the book, but it's worth browsing AV Expanded for ideas - I only took some choice pieces of the festival and a little of the expanded roseguard lore directly from it, but it helped with fleshing the story out.
Speaking of fleshing things out, it's worth finding your characterisations of the residents of Otari early, as they are you primary source of roleplay and if you establish them early it's pretty easy to get the party to start engaging them at every opportunity. Even though the story won't really deal with them in the last book, there's still plenty of reasons for them to seek out various services or advice from specific NPCs if they already know they exist.
The only thing I completely rewrote was Carmen Rajani and the Cooperative Blade. I think as written he's a sympathetic character who yearns for purpose, and letting the party work to redeem him makes for a better story and bigger moral dilemma when Urevian requests his soul (have Urevian meet the party and make the offer almost as soon as they get to his floor, so every time they go into a fight they know they could just sacrifice him and skip the floor). As part of that redemption I also made him one of the keys to unlock the lower levels instead of the Cooperative Blade (Vol settled down and hung up her sword immediately after defeating belcorra, so I decided it was her family that carried her spirit rather than the sword), which my players found corny but still preferred to the original plot when I told them about it later. He's way better as a morality pet than irredeemable dickhead.