Bosnia and Herzegovina is a second largely Muslim state in the Balkans, with most ethnic Bosnians traditionally professing the Muslim faith. However the Bosnian, Serb and Croat populations in this area are densely intertwined historically, aggravating racial tensions in the Bosnian War of 1992-1995.
The Ottoman empire was not (through most of its occupation of the Balkans) a strongly proselytizing empire. For instance, though the Serbian Patriarchate was abolished in 1463 following the death of Patriarch Arsenios II it was re-established by the Ottomans in 1557. This disruption was thus less than 100 years. Further, forced conversions were not generally required of the subject populations in the Empire, rather non-Muslims suffered from additional taxation and the loss of some rights making them second-class citizens.
Your chronology is also exaggerated. Although the bulk of the Balkans was conquered by the Ottomans in the half-century immediately preceding and following the capture of Constantinople in 1453, much of that area had again obtained independence less than 400 years later. Greece obtained independence by 1832; Moldavia and Wallachia were independent by 1821 and 1848 respectively and unified as Romania in 1859. Serbia was abandoned by the Ottomans by 1867.
Interestingly, the reason for the conversion of the Albanians to Islam remain unclear. What is agreed is that the conversion primarily occurred late in the period of Ottoman rule; Catholic Albanians mostly converted in the 17th century, and the Orthodox Albanians mostly followed in the following century. Primary source documentation is scarce, hindering research efforts to determine causes and motivations. The country itself is today 59% Muslim, 17% Christian, and the rest other.