i meander through here (only sometimes with purpose)

Star is too dim? No, Keid is dimmer than Sol but it is still a bright, cheerfully orange star. Atmosphere is too dense? I mean if it just kept going and going and going sure, but it doesn't. There's a rocky surface down there. Venus has roughly triple the atmospheric pressure of Erid, right? Per the measurements taken in the 70s, the surface of Venus is still receiving somewhere between 4,000-14,000 Lux. It's not pitch dark. You can still see when outside under a heavy, thick thunderstorm, right? You know your eyes can navigate a sidewalk with access to literally 1 Lux, right? Is Erid's atmosphere really that much thicker than Venus'? Or something in the atmospheric composition just absorbs that much red-orange spectrum light? Something that isn't a clear gas like ammonia and nitrogen? Is there just supposed to be that much water vapor? Something like nitrogen dioxide?

I have no issue running with the idea that their evolution zigged towards sonar instead of zagging the way ours did (early on) towards eye spots to notice day vs night. Evolution is messy. But to me, that gets framed in the novel as being due to the atmosphere more than the (ridiculously) short planetary rotation. Having distinct circadian patterns isn't as useful when the experience is closer to someone standing there flicking the switch of some dim, ambient mood lighting on and off.