I find it's better to take Gerogis after Tanais, as it's closer and means I don't have to split up my army.
Also, I never fight fair. In any battle, I have my horse archers skirmish and fire arrows, and when they run out, I run away to conserve casualties.

In my most succesful game, I never took Gava-Yugra or any of the northern provinces, I just kept pushing west and attacking the least defended settlements I could find.

It worked great, but one day I went to load the file and it CTD'd, and ever since then I've been foolishly trying different things.