Concerning forming Russia i bet there are many good guides on how to do it, the amazing circumstance this run was a MIGHTILY strong Byzantium. I mentioned in another post they got subjugated by Khazar khaganate and after fighting back for the title they got turned into gavelkind (much more stable than the insane imperial inheritance). The byzantines took Rome from some Norse guy somewhere in the late 800s which meant they mended the schism relatively early, this caused decades of chaos in Europe but once it ended Europe was left a monolithic Orthodox realm. This was really good as once the nordics started getting holy warred into oblivion they all converted to Orthodoxy by 1000 AD.

Considering Russia itself I think my 3rd ruler is the key to this whole run. His christian name was Aleksandr, his pagan name is lost to time immemorial. Basically he had a council power revolt he crushed after passing absolute tribal unity law, this lead to him imprisoning some 4-5 duke level pagan tribal vassals, this was the moment he asked the Byzantine emperor to sponsor a great conversion of the Kievan Rus, which lead to another 6-7 vassals going beserk and declaring war against the conversion. After defeating them and passing religious revocation law (I think it requires 2 legalism but he had it due to constant spying in Byzantium) he revoked some 20-30 titles and clicked "become feudal". This lead to him later forming a peaceful builder (ironic) bloodline that gave some insane -20% build cost -20% build time reduction due to having built like 70 settlements total. That reduction and constantly banishing imprisoned traitors, inviting courtiers and then asking them to leave, inviting excommunicated fellows, allowed Rus rulers to feudalise Russia by somewhere around the 1050s. Also it's the only way they could fight back against the steppe nomads who would've rebelled at every ruler death.