I have suddenly had a moment of inspiration: We can actually represent the Saka and the Sarmatians with the same factions and still have them separate (like a Steppe Superfaction). Basically, my plan involves removing Jaxartes and then adding Saka Haumavarka in the same place (because if you just rename it, there is a strange bug if you put a family member in there, according to desc_strat), Then you move Tribus Sakae south and make it into Saka Tigrakhauda, north of Parthia. BTW, I got the names from wikipedia, which got them from a Persian inscription by Darius in Persepolis and Ectabana.
These three provinces would all belong to the Sarmatians. The landblock would be lifted. Campus Alanni would also be moved south and west to make sure it is far away from the Sakae.
I volunteer to try this out, if you think it is worth it.