The S-Iberia thing is actually a two-sided issue. That Western colonizers (not only Iberians, I've already seen France, England and even freaking Brittany in Siberia...) want to colonize there while there is room left elsewhere is one thing ; that Russia isn't fast enough to claim the place, or that nobody else does when Muscovy fails its early game (which isn't an especially rare outcome), is another. Even if Western AI stopped prioritizing Siberia as much as they do, that wouldn't prevent them from going there in the late XVIIIth century if there still is nobody.
In my opinion, not only should there be a fix to Western colonizers' AI, but there should also be other continental contenders for Siberia. I am kind of unhappy when there is a huge and stronk Uzbek or Oirat dominating the steppes that's stuck within the ugly borders of the Siberian khanate. Steppe nomads, Russian nations other than Muscovy, and Perm come to my mind as nations who could be given a colonist to reasonably colonize Siberia. It would make much more sense than Iberian, French or English Siberia.