I came across this interesting historical text called the Cäğfär Taríxı, but it seemingly has thirty different ways of spelling the title. Anyways, this is a compilation of the history of the Volga Bulgars and has some interesting material on their ancient history as well.
However it seems that this text does not come without controversy, as it has a dubious origin story and the original manuscripts, composed in the 17th century, were supposedly destroyed in the 1930s.
I can barely find any information on it outside of fringe websites, but my googling is limited to English, perhaps the story would be different if I could speak Russian or one of the Turkic languages.
So for your steppe historians out there, what is general consensus regarding these texts? Does the historicity of these annals check out? Was it genuinely composed in the 17th century? Or is this a forgery?
Apologies for linking this propagandist pseudo history website here (serious warning here folks), but this was the only place where I could find an English version of the Djagfar Tarihi:
As a disclaimer, I'm not incredibly familiar with this area, but questions of national origin and identity, public archaeology, and historical forgery are always interesting!
A short answer: no, most scholars consider the Djagfar Tarikhy a forgery. Uli Shamiloglu, formerly at Wisconsin, writes:
I claim that this work is undoubtedly a complete falsification, perhaps, continuing the heritage of earlier Bulgar falsified history. (63)
The problems of the text, "historical issues, including anachronisms," are "too numerous to illuminate in this essay" (66). Some issues in particular are the conversion of Hijra dates to Gregorian ones, and anachronistic ethnonyms ('Tatar' and 'Bulgar'). And, ultimately,
I can't even imagine that any of historians or Turkish [scholars] would spend their time on this. (66)
Those that do attest to the authenticity of the piece aren't exceptionally reliable. One of the proponents is Yuri Begunov, who is primarily famous for a variety of nonsensical, anti-Semitic conspiracy theories. Others seem to be, from what I can tell, invested very heavily in constructing a particular national history (like you noted); Shamiloglu references Mirkasym Usmanov, for example. If you want a real rabbit-hole, look into the New Chronology of Morozov and Fomenko.
There are several other forgeries that come to mind, chiefly the Veles Book (early Slavic religion) as well as replicated or imagined artifacts (the Tiara of Saitaphernes, purchased by the Louvre). One of the great difficulties when creating compelling forgeries is in writing obscure or unattested languages - if other texts in the language exist, you have to replicate them, and if they don't, you have to know, roughly, what they would have been. Another example of this is the Oera Linda, a Frisian nationalist forgery, and the supposed writings of Jacob of Ancona, the City of Light.
As far as spelling goes, transliteration from Russian isn't incredibly standardized depending on who is doing it and why, while Tatar Cyrillic (generally?) uses Yanalif transliteration from the Soviet era.
Shamiloglu, Uli. "" Джагфар Тарихы" Как изобреталось булгарское самосознание." Россия и мусульманский мир 11 (2012): 57-68