Yuri Vsevolodovich fought Bulgar and founded Nizhny and was preparing for new
campaigns...
As soon as Kazan (which replaced Bulgar) fell in the 16th c. expansion became
really rapid.
Well I guess we both agree that in the world with butterflied Mongols the key element of preventing Rus eastward expansion is independent Volga Bulgars.
I wonder what makes you so sure that the Volgar Bulgars were doomed against the Russian expansion.
Volga Bulgaria was a flourishing civilization. According to some historians, over 80% of the country's population was killed during the Mongol invasion. The agricultural development suffered a severe decline and they say that this area never reached the level it had had before the Mongols.
As for me I think that the Volga Bulgars had all chances to hold against the Russians.
By the way you know that the Battle of Samara Bend (or the Battle of Kernek) between Volga Bulgaria and the Mongols in 1223, probably was one of the first battles in history which the Mongols lost.
That was when the Russians suffered only humiliating defeats against the Mongols.
So military the Bulgars were not that weak as you may think.
So - northern waterways, fur tribute, missionaries.
Northern waterways - may be so.
But you can not compare the eastward expansion of disunited Rus with the expansion of Ivan the Terrible.
You should read about the internal fights of Russian princes against each other - it drained their strength a lot. And the Cumans took part in it.
Edit:
By the way the Cumans did just fine against the Russ in OTL. In this ATL in the world without the Mongols they might expand into the Siberia as the Mongols did. And the Russians would meet a serious competition from their part as well.