Turkic History of Bulgars
I think It’s benefical to mention South Slavic peoples and languages first. There is no such thing (as it was mentioned in other comments) as “Proto-Bulgarian” language, all South Slavic Languages are a part of South Slavic Dialect Continuum which emerged from Old Church Slavonic in 9th century and are not separate languages, they are a dialect chain. Slavic migration to Balkans happened in mid 6th century, they were not indigenious peoples of Balkans (Bourchier, David, 1911).
Then we should discuss who Bulgars were. As we know, their language was Oghur Turkic (Clauson, Gerard, 2002) from Eastern Rome sources, we know their nomadic lifestyle, we also know their religion were the indigenous religion of Altai-Sayan, Tengrism until Boris I. We know that Turkic existence on South-Eastern Europe goes back to late Hun period (5th) century, again, from Eastern Rome archives but first usage of the name Bulgar goes back to Old Great Bulgaria which was established on Crimean Peninsula. (Waldman, Mason, 2006)
Last ruler of Old Great Bulgaria, Kubrat’s last will was his sons not to split and keep ruling Old Great Bulgaria which they didn’t obey. His five sons and their people splitted to five different entities and migrated to different regions. Biggest son, Batbayan couldn’t resist Khazar Khanate pressure and Old Great Bulgaria was collapsed and became a vassal of Khazars. (Zhivkov, 2015). Second son was Kotrak who accepted as establisher of Volga Bulgaria. Volga Bulgars converted to Islam in 10th century became more “Kipchak” (their language shifted from Oghur branch of Turkic Languages to Common branch of Turkic languages’ Kipchak-Bulgar sub-brach in time) and they eventually become Tatars while Chuvash people kept their language solid which eventually led Chuvash language to be the only surviving Oghur Turkic language. Third son, Alcek and his people migrated all the way from today’s Ukraine to Matese Mountains of Southern Italy. Fourth son, Kuber who became a subject to Avar Khagan and became the ruler of Syrmia under Avar regime. And the last son was Asparuh Khan who established First Bulgarian Empire.
The rulers and Turkic minority under still continuing Slavic migration to Balkans got assimilated in next 200 century. We can’t refer a Turkic existence after Boris Michael I, first ruler of First Bulgarian Empire who converted to Christianity and left the title “Khan” or “Kansubigi” aside, adopting Knyaz title (Golden, 2010).
(Clauson, Gerard (2002). Studies in Turkic and Mongolic Linguistics. Taylor & Francis. p. 38.)
(Waldman, Mason (2006). Encyclopedia of European Peoples. Infobase Publishing. p. 106–109)
(Boris Zhivkov, Khazaria in the Ninth and Tenth Centuries, East Central and Eastern Europe in the Middle Ages. BRILL. p. 138 & 228–229.)
(Peter B. Golden (2010) Turks and Khazars: Origins, Institutions, and Interactions in Pre-Mongol Eurasia, Volume 952 of Collected studies, Ashgate/Variorum. p.4)
(Bourchier, David (1911). Bulgaria and Language and Literature. Chisholm, Hugh (ed.). Encyclopedia Britannica 4 (11th ed.). Cambridge University Press. p. 785.)