This is one of the most immediately professional feeling submissions I've seen so far. Absolutely phenomenal job on basically all counts. The writing is great and very thematically appropriate. The layout is really well done. The art is striking. It feels immediately playable as a dungeon crawl and also has a clear through line for a story to be told.
The only real question I had was whether the GM should be tracking the dungeon era as a whole or if each room would be tracked independently. My initial inclination was to track it as a whole, meaning they would have the opportunity to double back and revisit areas that they saw in a different time and see how they're different, but that doesn't necessarily feel like it's likely to happen. I could see the potential for a fun time crystal macguffin being added to help free Yasha-Gok but also see the differences in the dungeon areas that the party has already visited if an era change occurs at a later time if you tracked the era as a dungeon wide effect.
If the era is a location by location effect, it would mean that players would generally only see one version of a location and if they happen to roll an era change, then they'd get to see how it changes immediately.
The only other thing I can point to that I think could be sharpened up a bit is the Explorer vs. Tribesman subplot. While it does feel authentic to the sorts of stories that came from that era, it does have some lingering implications that feel a little dated. I think I would prefer to see this as an Explorers vs. Devotees, which does seem to be the framing in the future era for The Library location. I think you set up that sort of a conflict in your introduction and I think dropping the Tribesman label in favor of worshippers of Yasha-Gok would just make it feel a little more timeless and drop some of the weird awkwardness that comes when you use labels like Tribesman.
Other than those two very minor points, this is definitely the cleanest submission I've reviewed so far.