if Germany was as self sufficient as you say they would not have given only half rations or had famines, they would not have imported more than 1 million tonnes of grain and almost a million tonnes of oil from the soviets in just 6 months, the problems it did have speak for itself and it was unpreparedYou heavily underestimate how Nazis built up self-sufficient sources for most of such materials.
They synthesized fuel, in so huge numbers that it carried them through war (not Romanian oil or anything like that).
They secured Norway to not get winter shipments of metals from Sweden (delivered across Norway shores in winter) disrupted or worse, get Sweden occupied/swayed to Allies.
They relied on Spain and other 3rd countries whenever possible to get resources.
They made rubber too.
They even heavily increased local food production, severely decreasing needed food imports and significantly optimizing food situation compared to WWI.
If anything, you underestimate how much German military economics was prepared to answer such challenges. And if not for 1943-1945 bombings with USA help, it would last much longer.
Except that the regime itself changed because of huge failures of government?
Surely Germany wanted oil, bit it didn't just occupy Romania. It also wasn't in awe from having to occupy Greece and Yugoslavia, but the former's regime changed forced them to do it.
Hello, 2nd battle of Kharkov wants to talk to you. The one where Axis destroyed a whole army and opened a path to the operations in Stalingrad direction. Here is a map which shows how they could advance after eliminating a whole army there:
Sure, resources and Stalingrad's strategic and symbolic meanings also played a role, but main argument was the fact that they could make a resultative offense there with strategic gains.
And more generally, they looted what they occupied indeed, but they didn't count on it that critically.
also, your last sentence contradicts itself as you said "strategic meaning also played a role" but also that the main argument was "strategic gains"
The picture you have comes from a Wikipedia page that advocates "The grand plan was to secure the Don and Volga first and then drive into the Caucasus towards the oil fields", so your missing the point of that picture which was to say they were pushing up to the Volga river, which you can just about see on that map, in order to get the oil in the Caucasus