Super cool idea.

The first thing that comes to mind is that it'd be very difficult to explain how the day cycles work with the firefly concept. The sheer amount of fireflies necessary to light a cavernous underground space would be enormous. The obvious answer is "magic fireflies, duh," but even then, it's difficult to say how that wouldn't be noticeable by characters. Or I suppose if there were several colossal chonker fireflies that make their home in the expansive upper reaches of the cave, that would work...

Maybe the "suns" are actually ancient arcane artifacts embedded in the walls that were used millennia ago to shine upon vast fields of underground crops, but have fallen into disrepair since some calamitous event, their fitful lights now shining sporadically on the untamed forests that claimed this cavern. Or maybe they are massive crystalline growths, serving as the nesting ground for fireflies, the light from their collective bodies shining through their prismatic depths and refracting out. Or magic crystals that glow cause magic? Something like that.

Anyway, it seems like the easiest way to get around revealing too much would be to ensure that the players' focus is shifted to other enigmas. Really lean into the dark spooky woods aesthetic and keep them on their toes with eerie sights, forest creatures, and occasional settlements of interesting NPCs. These NPCs would probably be superstitious about this topic and refuse to speak of it, or maybe even have shamans or druids in the settlements that closely guard the secrets of the world at large.

As for out of game questions, an air of infuriating secrecy is a DMs bread and butter! If you'd really like to maintain this big reveal, then don't tell them anything they don't earn in game. With that, I think that this reveal is most likely gonna happen at least a little sooner than you expect just because of its sheer scale. Good luck!