Although legally 'Austria' is a somewhat amorphous term throughout the 18th century, it is commonly used to describe the whole of the Hapsburg domains that existed within the boundary of the Holy Roman Empire. This in fact consists of the Archduchy of Austria in addition to the Kingdom of Bohemia and a number of more minor states in complex personal unions with each other.
Matters are complicated further by the fact that the Hapsburg Archduke of Austria was also Holy Roman Emperor, but the authority of that title had become almost meaningless by the 18th century. However, the Emperor retained more power relative to OTL, because Prussia's rise to prominence was largely averted and no other German state was powerful enough to challenge Austria directly. In the 1770s and 1780s, Hapsburg power grew to the extent that the Emperor could be said once more to, at least in some ways, command all of Germany: yet that shortlived unity blew apart in the Jacobin Wars.
Austria fought in most of the wars of the 18th century, and in 1783 peacefully acquired the Electorate of Bavaria when its last Wittelsbach ruler died and the heir, Charles Theodore, conceded the throne in exchange for the Austrian Netherlands, which became Flanders.
In the Jacobin Wars, Austria was Revolutionary France's main foe initially, and took the brunt of the Republican armies, almost being destroyed in the process. The fact that the Austrians barely managed to survive themselves, and were then jumped upon by the opportunistic Ottoman Empire, meant that the French were able to run riot in the rest of Germany. The German states therefore became increasingly resentful of the lack of Austrian protection, especially the Imperial Cities and ecclesiastical states that were mediatised by their more powerful neighbours in the name of defending them from the French. Thus the death of the Holy Roman Empire was indeed sealed by Emperor Ferdinand IV's outburst in the Reichstag in Regensburg.
Note that because the Emperor was elected by the Reichstag, in cases of succession dispute there may be periods with an empty throne.
House of Hapsburg
1711-1740: Charles VI
House of Wittelsbach
1742-1745: Charles VII Albert
House of Hapsburg-Lorraine
1745–1765: Francis I
1765-1781: Joseph II
1781-1793: Leopold II
1793-1798: Ferdinand IV
1798-1830: Francis II (Archduke of Austria, unelected claimant Holy Roman Emperor)
1830-1883: Rudolf III (underage with Regency until 1843)
1883-????: Ferdinand V
National flags were not widely used in Germany in the 18th century. The Holy Roman Empire was best known by the double-headed eagle banner:
While the flag of Austria and the Hapsburg dominions used the same colours as a livery bicolour: