You mean Latvia, Lithuania, and Estonia?
Are you suggesting that the best move here is to create a UBD consisting of Latvia and Estonia while creating a separate independent Lithuania?
I don't think that the USSR would have been anywhere near as aggressive in this TL due to their fear of a powerful Germany, though.
Also, how wealthy were the Baltic states in comparison to Poland in the early 20th century?
Having Ukraine fall back into the Russian/Soviet sphere would probably be fatal for Germany, though.
It would comprise modern day Estonia and Latvia, Lithuania should always remain independent since it had far less, if any, German influence or affinity. My understanding would be that a German Duchy forms with these areas dominated by the "German" elite and exiled Russian nobility (and middle class too likely), incorporated into the German Empire as a new state with representation in both the Bundestag and Reichstag while Lithuania is established as its own state under Imperial German influence. I think so long as the USSR or even a revanche non-White Russia exists then it has a chance of remaining part of Germany or more likely gaining independence while aligned to Germany. It is what I see likely for Finland, a new German ally, not fully a satellite or client state but definitely dependent upon trade and military support. That is what I might argue happens with Lithuania too.
In my opinion the USSR must be "aggressive", after all it is the font of world revolution, but I agree that it would not necessarily put ideological rhetoric above practical reality. Under Stalin the USSR is a dangerous schemer looking to exploit weaknesses, but under a Trotsky firebrand it will be more reckless, thus we saw how Trotsky was sidelined and for me would be here just as in OTL.
I think that the Baltic Duchy should be on par with eastern Germany in short order, it had comparatively good infrastructure and education, it has a German speaking minority population and both ports and relatively easy connections to the German economy. Agriculture likely dominates but some improved industrialization should follow as the German economy overall recovers. Longer term I think this Duchy may seek independence but by then the political character of Germany might be liberal enough to make it a truly velvet divorce.
In my mind Poland fairs a little less well under the German shadow. First I think it is easier for Poles to migrate to jobs in Germany and the investments will be lighter from Germany than was from France to the independent and "hostile" Poland. Besides Poland is a frontier and the bias is to keep strategic industries closer to the core. That said I think Posen and Silesia get more investment and add more Poles who would join a German Polish minority community, benefit from closer distances to home in Poland and overall feel quite portable between an aligned Germany and Poland. In my mind an independent Poland should fall under the A-H influence, Germany certainly would feel entitled to it as a satellite but a smarter policy would be to let the Austrians tangle with the Poles and remain more distant, it might keep the Polish minority less angered over things and dampen calls for Germany to give it territory too. Instead I think Germany should have pursued influence in Ukraine.
To balance the power in Europe I certainly want an independent Ukraine, without it the USSR is far from being a dominant power, but I fear reality is that neither Germany nor A-H can hold it during the turbulence of a Russian Civil War. It also attracts the Whites who further tip it away. I am not convinced Ukrainian nationalism is strong enough to keep Ukraine independent. In my opinion the USSR was a peer of Imperial Germany and over time would grow in power to be somewhere between her and the USA (and British Empire should that also survive). But Imperial Germany will be pushing not less than 1/3 if not 1/2 the economic and military might of the USSR, further she has no Warsaw Pact and potentially no PRC, worse we might further have an Imperial Japan to the East, all told the USSR will be more alone in these ATLs than ours so its true power should not be overwhelming. I admit this shades things to a multi-lateral world using some breaks in favor of the CP and against the USSR but not so implausible as to be fatal.