I believe Ukraine is being set up as a political and economic satellite anyhow. The overwhelming majority of Ukrainian wealth and resources are going to be sent on a train straight to Germany. The only Ukrainians with any political or financial weight to throw around meaningfully will be German collaborators. If this pisses the Ukrainians off enough it gets tricky, because if they kick out the germans, they could go it alone or accept becoming a union republic of the USSR with autonomy and certain rights. It’ll be a hell of a fight though either way because Germany has their own wealth, and that of Ukraine + the other puppets they’d have been “borrowing” from for a while at that point. Any Ukrainian separation from Germany will be starting three steps behind, and as long as Ukraine remains a puppet, it will not be able to pursue any independent foreign policy without Germany’s permission.
The thing seems to be that you let yourself be guided more by a propagandistic distorted image of Germany in your thought experiments than by a real Germany, especially if you consider the different distribution of power ITTL.
ITTL the Emperor was forced into a kind of proto-British system, which means that he can no longer act without having the Reichstag in mind. In the Reichstag, the SPD is currently the strongest force and thanks to their cooperation with the Centre they have a majority of votes.
Both parties have no great interest in colonial adventures and will not support policies that involve Germany in such things.
This means that in addition to the restrictions based on strategic considerations, there are further political restrictions that limit the actions of the Imperial government.
The strategic considerations with regard to Ukraine and, to be honest, with regard to almost all states in Eastern Europe, with the exception of Poland, look something like this in my opinion:
First, these countries are to serve as buffers and bulwarks against Russia and the Bolsheviks, respectively, which forces a certain flexibility from Germany in how far they will influence local politics.
I think that from the German side there will only be a handful of demands on these new states, namely membership in Mitteleuropa and a monarchy as a form of state with a German prince as head of state. How much power this prince will have and how the state is otherwise structured will not really interest Berlin.
Secondly, Germany is interested in the local mineral resources and the products of agriculture, the latter especially to negate the benefit of a second blockade.
This is one of the few areas that would be feasible in a colonial relationship, but market mechanisms in the framework of Mitteleuropa should be good enough for that.
Thirdly, I assume that these states should serve both as targets for the export of German goods and as targets for investment, as they already were before the war.
This would also be possible in a colonial relationship, but Germany has already made the very costly experience that colonies are really not suitable for this.
Therefore, I do not expect Germany to interfere too much in internal affairs, except of course to put down a Communist revolt, since Germany can achieve more or less everything it wants without expensive colonialism and can thus present itself as the guarantor of the independence of these new nations from Russia.
And that would be something that could find support in the Reichstag.