One interesting thing about the River spirits, in the old lore article, river aelementors are noted as more common (https://www.warhammer-community.com/2021/01/26/how-well-do-you-know-the-spirits-of-hysh/). Instead of one large spirit, is it possible we could see packs of water spirits?
Now that also could just be reading into it more as well.
Edit: Perhaps the lumineth warband actually contains smaller aelementors?