Here I don't agree decolonization is an inevitable process. In OTL, it was, but in this one, with such POD, things could go in any direction. Populations won't necessarily rebel against an empire or an authoritarian regime. Even in OTL we have examples of those two.
The problem isn't just a matter of authoritarianism.
The problem is
sovereignty. Being a colonial province ruled by a governor appointed by a bunch of foreigners who live in a different freaking hemisphere is not a good condition to be in. Remember, that governor had a life and a family in the metropole before they were sent out here, and they are likely to eventually return. Their priority will be governing the colony in a way that pleases the elites back in the metropole, whose opinions and good feelings will decide their own future. Their priority will
not be governing the colony in a way that is beneficial, respectful, or helpful to the citizens of the colony. That may happen by happy coincidence, and in some cases the colony and metropole's interests may align, but when the chips are down... a foreign-appointed government you have no say in will almost always prioritize its welfare over yours when there is a conflict of interest.
And since the colonial government has the full power to legislate the shape of your economy, we see
very consistent policies designed to reshape the colony's economy for the benefit of the metropole. Such as deliberate de-industrialization. Such as the engineering of infrastructure to connect the mines to the ports and not much else because the colonial governor in his spiffy new pith helmet has very little reason to invest in much else. Such as favorable access to labor and land for corporations based in the metropole. And so on.
...
Now, there's a way to solve this! Give the colonies political representation in the metropole, or give them sovereignty. "Sovereignty" is the process the British took with the dominions, but... bluntly, that only proceeded smoothly because the dominions were full of white people and could lobby for a reasonable degree of legal autonomy in London and expect to be treated with respect. The idea of dominion status for India, for example, was never something Britain would credibly agree to without a major struggle, and this is a large part of why Indian self-rule movements
quite sensibly focused on independence, and had good cause to do so.
Giving colonies political representation in the metropole is
theoretically an alternative, but in practice was very rarely considered seriously for reasons that should be obvious on brief reflection.
I do not see a reason to think that an ATL Germany that won World War One would be uniquely and amazingly different in such a way that would enable them to square this circle. We can say "it could happen," but to me it smacks of motivated reasoning, as if history is somehow going to be "better" if Namibia is governed from Berlin rather than Windhoek.
...
Non-imperial states, even autocratic ones, don't have the same problem. China is an autocratic state, but the great majority of the country is "all metropole. Party officials and government leaders and other prominent people can come from almost any part of the country, and the nation is governed by people drawn from a mix of different parts of the country, so there is always at least
some incentive to rule any given area with an eye to its actual long-term benefit, not just as an extractive pinata to hit with a stick so that gold and cash crops come out.
Now, places like Tibet and Xinjiang have the same problems that an exploited colonial province does, in that Tibetans and Uighurs and so on have basically no access to the Chinese power structure. And, lo and behold, they experience the same bad things that nearly every colonized population in history has experienced: settlers displacing them, being criminalized, run through camps, degraded, and so on.
Because this is something that happens over and over and over in far-flung empires ruled from a central metropole. It is
really hard to think of a way to beat it.
They can't defeat the Empire. What they can do is defeat the BEF, ensure the British have no path to victory over Germany and hold territory that represents the most dire threat to Britain's long term security, in particular the French Channel coast ports. In such a case it is Britain's interest to make a deal to bribe Germany away from the Channel ports, it's better to lose colonies than to have the worlds second largest navy and most powerful Army stations 21 miles from your shores.
Maybe that's the theory, but it didn't work against Britain during the Napoleonic Wars 120 years earlier and it didn't work against them during World War Two twenty years later, so I'm not sure it would work in the late 1910s or early 1920s, either.
If anything, in that position as the Germans, you'd have the worst of both worlds.
You're still trying to threaten a cross-Channel attack. But unlike Napoleon, your troops rely heavily on machine guns and artillery and so cannot put up a credible offensive threat in an amphibious landing that isn't heavily supplied by sea. Twentieth century armies have orders of magnitude more ability to smash what pops up in front of them than their nineteenth century predecessors, but the price of that is needing much heavier continuous flows of supplies that cannot be replenished by pillaging the countryside.
And unlike Hitler, you don't have the advantage of a meaningful air force capable of at least giving the British defensive positions on the other side of the Channel a hard time. You have a big fleet, but to use that fleet in the Channel means bringing it into very narrow waters where smacking into a bunch of mines or unexpected torpedo attacks could put a big dent in your fleet in a hurry, and the enemy will be only too happy to exploit that dent to their advantage.
If the submarine blockade failed bring Britain down in that situation (and it didn't in real life), I'm not sure that holding the French Channel coast would do the Germans much good.
Sure, you can say "it could happen," but at some point that's just papering over the cracks in what is normally a very ugly edifice.