I think both are valid. The original universe of the king in yellow and the universe built by others that give more detail on Carcossa. While I prefer the original universe where we know basically nothing of the contents of the in universe book ‘the king in yellow’ and subsequently what even is in carcossa or what carcossa even is, I think it’s OK to fill in the gaps in the canon. I’d recommend reading the king in yellow as a separate story from the grand universe though because it hugely relys on the effect that the unknown has on us humans. All the information and context the subsequent authors added take away from that and the original author gave an incomplete account of the universe on purpose.