As I noted in my previous answer you linked, it is noticeable that a rather large number of Nazi and Wehrmacht occupation personnel in the Balkan theater including the troops were of Austrian origin. To list just a few: Edmund Glaise-Horstenau, Wehrmacht commander in Croatia; Franz Böhme, commanding general of the Wehrmacht in Serbia; Lothar Rendulic, commander of the 2. Panzer Army in Yugoslavia; Alexander Löhr, OB South East and commander of Heeresgruppe E; and the list goes on, including up to Hitler himself who was Austrian and had spent several years of his adult life in Vienna.

This is no coincidence and had very much to do -- in case of generals, troop leaders and soldiers -- with them being from Austria and the Austro-Hungarian monarchy. The Nazi attitude and view of the South Slavs / the Balkans was very much influenced by the attitudes and views of the South Slav population of the Habsburg empire. Throughout the twilight years of the monarchy up to WWI, Austria-Hungary was very much entrenched in national conflict between the various nationalities with Slavs trying to gain greater recognition and autonomy similar to the Hungarians and the German-speaking nationalists trying to retain their dominance at least in their part of the monarchy.

The South Slav population were viewed with an attitude that carried over into Nationalsocialist ideology and that Balkan scholar Maria Toderova dubs Balkanism. Toderova formulated her theory of Balkanism on the basis of Edward Said's concept of Orientalism. What it entails is that the population of the Balkans, similar to the Orient in Said's theory, is by Western Europeans and their discourse constructed as an essential other. We can observe this phenomenon rather frequently and even up to the 90s. One part of it is that the inhabitants of the Balkans, similar to the Ottomans and other people of the "Orient", are viewed as naturally inclined to violence, savage, brutal, having been in conflict with each other and the world for a long time, having a tradition of brutal banditry (without understanding the actual reality of the Hajduks and similar phenomenon), and generally being unreliable and treacherous. This view, shaped heavily by travel reporting of the 19th century and generally more romantically inclined idealizations (in the positive and negative) of literature of the same time. As I said, it this concept is still very much present and if you think back at the Balkan conflicts of the early 90s, you get a good sense for what it entailed.

Anyways, this whole discourse was of course massively radicalized through the First World War. The narrative of Serbian aggression paired with a propaganda that painted the Serb as brutal and savage Balkan beast was also extended to the empire's South Slav population. Viewed by people like Conrad von Hötzendorf with a sort of general suspicion of being in league with the Serbs and the Yugoslavist movement, the Austrian-Hungarian monarchy deported at least 150.000 of its own citizens to camps in the Austrian Heartland such as in the Katzenau Camp near Linz or Thalerhof near Graz. In the latter camp, about 1.500 people did not survive the first winter of the war and basically were left to freeze and starve to death. All because they had become politically suspicious due to their nationality.

With the end of the First World War, these stereotypes and prejudices against the peoples of the Balkans were further worsened due to the fact that not only did the German-speakers of the empire develop their own "stab-in-the-back" myth not only pertaining to people like Social Democrats and Jews but also including the South Slavs of the empire. This further radicalized when during the first years of the Austrian Republic, the newly created SHS state (the later Kingdom of Yugoslavia) invaded territories in the south of Austria, in Carinthia and Styria. This conflict, fought on the Austrian side by the predecessor of the later Austrian army as well as the Austrian equivalent of the Freikorps was a major catalyst in radicalizing Austrians into international networks of radical and extreme right wingers (many of them struck contacts with the Freikorps and the reactionary movement in Hungary against Bela Kuhn's Soviet Republic) as well as in advancing a negative racialized view of South Slavs. The today mostly forgotten conflict was vicious because Austrian troops once again employed a general suspicion against the --mostly Slovenian -- South Slav inhabitants of Southern Carinthia and Styria. How radicalizing this conflict was, can be traced through its participants. Hanns Albin Rauter, the later SS and Police leader in the Netherlands, wrote that it was this conflict that made him a Nazi. Odilo Globocnik, later commander of the Aktion Reinhard death camps, participated in the conflict in a student battalion. Franz Böhme, Wehrmacht commander in Serbia, as well as his most vicious commander, Walter Hinghofer, both fought there. It can be stated with confidence that this conflict is the Austrian equivalent to the German Freikorps for many later Nazis.

Now, as you rightly point out, the Balkans were in the initial conceptualization outside of the German vision of "Lebensraum". The attack on Yugoslavia in 1941 was not one that was planned in the long run and the set up of occupational policy was influenced by improvisation and the need for troops for the attack on the Soviet Union. This is the reason why for example the Ustasha was allowed to rule in Croatia despite them not being the first choice for collaborators (that the was the Croatian Peasants' Party but they refused collaboration with the Germans) but in the case of Serbia, the decision to place the territory under the administration of the Wehrmacht was very much influenced by ideology. "The Serb only understands the language of violence" ran the the thinking among high ranking Nazis and due to the Serbian proclivity for Partisan warfare and banditry, they had to be reigned in.

The occupation of Yugoslavia was a particularly brutal one, even within the scope of the Nazi occupation of Europe. The country lost about 10-13% of its total population during World War 2 due to Nazi and Ustasha violence. A lot of that came down to the brutal policy of reprisals the Germans ran in Serbia and elsewhere in the Balkans. When the Partisan uprising by the communists broke out in Serbia in 1941 (described in more detail in my post here), the German army decided to shoot 100 civilians for every dead German soldier and 50 for every wounded German soldier. They toned this down a bit by the end of 1941 but between October, when the policy went into effect, and December 20.000 civilians, including the country's 5.000 male Jews, had been killed. This move was motivated by exactly the ideology described above. Because in the eyes of the Nazis, the peoples of the Balkans were savages -- something only heightened by Ustasha violence in Croatia -- the Nazis and the German Army felt they needed to treat them this way and went on a brutal killing spree throughout the country.