I don‘t see any particular reason why Nazi Germany would necessarily have to collapse or fall into civil war after Hitler’s death. If China can survive Maoism, with all of its horrific and in some cases downright bizarre manifestations (including real anti-intellectualism), then I see no reason why Germany couldn‘t survive Hitler. I mean, it did survive Hitler, despite the fact that the country was a burnt out ruin that lost millions of people and a third of its territory in the wake of WW2. (I think Germany and Japan‘s post-war rebirths were nothing short of miraculous.)
I‘m also dubious about the prospect of a never-ending partisan war in the east. I would argue that the Nazi’s utter lack of moral scruples when it comes to the treatment of enemy populations would actually be quite helpful in this regard. There can be no partisan war in the east if all the people there have either been killed, starved or deported. And genociding whole populations isn‘t as hard as it might seem. I mean, as far as I know there was no partisan war in the Ukraine during the Holodomor either, and I suspect Nazi policy in the east would basically be a Holodomor on steroids. I also see no reason for any protracted and sustainable rebellions in the occupied territories of western Europe (France etc.). There weren‘t any such rebellions in OTL (at least prior to the Allied invasion of France), so I wouldn‘t expect them in case of a Nazi victory. There might at one point be equivalents to the OTL anti-communist uprisings we saw in Hungary in 1956, or Poland in 1981, but I‘d expect their fates to be similar.
I think Hitler would probably die sometime between 1945 and 1960 at the latest (let‘s say 1955), so by that time he will have most likely have made arrangements regarding the succession. I remember hearing some time ago (I think while listening to some podcast) that he actually didn‘t want to be succeeded by another ‚Führer‘, but that he wanted the positions of Chancellor and Reichspräsident to be separate again, to be filled by different people. Apparently he wanted his successors to rule in a more collective manner, not too dissimilar from party rule in the USSR and other communist countries post-Stalin. I do think there would be some kind of power struggle, but I think it would look more like what happened after Stalin and Mao‘s death in the USSR and China, instead of civil war.
Furthermore, I would expect the long-term economic foundations of a victorious Nazi Germany to be much stronger than that of the USSR or other communist countries. The Nazi economy wouldn‘t be a top-down, centrally planned command economy that runs every single business and factory in the whole country, at least during peace time. There would be lots of interventions, subsidies and controls, and key sectors of the economy might be (partially) state owned, but overall the system would still be based on private ownership. There would be no collectivization of agriculture, for example, and private businessmen would still be able to accumulate (moderate) amounts of wealth. This is why I believe that the Nazis (and fascism in general) would have been a much more formidable enemy for the US (and liberalism in general) in a hypothetical alternate Cold War, more than communism ever was. I‘ve seen it often mentioned that the Nazi economy wasn‘t sustainable in the long term, but I‘m dubious; I see no reason why those problems couldn’t have been solved through economic reforms, especially if the Nazis have all the resources and vast markets of Europe at their disposal. To give a current example, people have been predicting the collapse of China for quite some time now (there‘s even a book from 2001, called The coming Collapse of China), but it doesn‘t look as if that‘s going to happen anytime soon.
I would also expect that post-war fascism would over time undergo an ideological leftward turn, and that it would absorb a lot of the revolutionary energy that in OTL was utilized by communism. With Europe conquered, Bolshevism defeated, and the Jews either dead or deported to Madagascar (or somewhere else), the Nazis would eventually need a new raison d'etre. I think they would adopt an increasingly anti-capitalist and anti-liberal stance, in opposition to the US and its allies. They probably would focus on things like environmentalism, healthy living, robust social safety nets and strong communal bonds, while claiming that the liberal system is detrimental to those things. As for the anti-intellectual aspects of Nazism (which mostly came down to opposition to ‚Jewish Science‘), I think much of that would eventually moderated or abandoned, just like the Soviets abandoned Lysenkoism (which claimed genetics was fake, bourgeois science) after Stalin‘s death.
In other words, I think it‘s conceivable that a victorious Nazi Germany could very well survive to the present day and beyond. Maybe Hitler would even be regarded (slightly) critically by his later successors, in the same way to how Mao is regarded in China today, where the official position of the communist party is that his policies were 70% good, while the other 30% were… not so good, lol. (Though I admit that‘s probably unlikely.)