Of course. But they were subjects of the Khazars and were too few in number to found a state on the scale of the Ottoman Empire, especially located there.
That
is true, but keep in mind that the Khazars were crushed by the Rus' under Svyatoslav in the 10th century, and that the Volga Bulgarian empire existed until the Mongol conquest, which was completed in 1236.
Volga Bulgaria also had a number of relatively large cities, like Bolghar, Bilär, Suar, Qashan and Cükätaw.
And some sources say that Bolghar at its height rivalled Baghdad in size and wealth,
although this could very well be an exaggeration...
But nonetheless, the trade routes to Central Asia, China, the Middle East, the lands to the North, and the rest of Europe gradually fell out of use when
new trade routes to the East were unlocked during the Crusades, and even before its conquest by the Mongols, the Kievan Rus' and its Russian successor states (especially Muscovy) were forcing the Volga Bulgarians eastwards with their raids and conquests.
...and of course you're absolutely right that - even at its height - Volga Bulgaria was not nearly as powerful and influential as the Ottoman Empire.
Yes, the nowadays Bulgars are Europe must be counted as Slavs. However, they were slavified* - the original Bulgars, as they came into Europe, and the Volga Bulgars, were turkic.
*Or maybe rather, the Bulgars came into a then already largely slavic area, and became assimilated by then, so that most "Bulgarians" do not descend from the Bulgars of then.
This is absolutely correct; the original Bulgarians were a Turkic people, yet those that crossed the Danube and settled there intermarried with the large Slavic population that had settled here during the invasion of the Avars.
However, the Tatars are the Volga Bulgars? I thought they came to Russia as part of the Mongolic Invasions, another nation.
This is a little complicated: the Tatars indeed came along with the Mongol invasion and shortly afterwards.
(in fact: "Tatars" was the name commonly given to the Mongol warriors, wether they were of Mongolian or Turkic descent)
And the Volga Bulgarians had their own identity and ethnicity, and they were clearly different from the Tatars that came
with the Mongol invasion.
However, Volga Bulgaria (which was an independant state when the Mongols conquered it) was pretty much destroyed when the Mongols conquered it, and the area saw an influx of Central Asian nomads of both Turkic as well as Mongolian descent.
After the Mongol conquest, Volga Bulgaria became part of the Khanate of the Golden Horde,
which became a muslim state when the convert to Islam Berke Khan came to power in 1257.
And during Berke's rule, most Mongolian and Turkic nomads of the Golden Horde converted to Islam as well, and because Volga Bulgaria had been a muslim state since the 9th century, the muslim Tatars had become pretty much inseparable from the muslim Bulgars (after all, they shared not only their faith, but much of their language as well).
And therefore, its not quite suprising that both groups became known as Tatars during and after the times of the Golden Horde, even though the Volga Bulgarians remained a separate group of Tatars: the Kazan Tatars.
