1918: The social democrats, after the old order has fallen anyways, decide to side with the communists. Opponents of that move are sidelined. The German brand of communism is slightly less radical, also thanks to social democrat participation, slightly more democratic - even though many right wing parties are outlawed - and considerably more humane.
1919: The German communists take a much more scientific approach to development than the Russians, leading to some equally desastrous experiments, but also to some great advances in science, technology, industrialisation, automatisation, and so on. Quite a few other central European countries turn communist.
1920: Thanks to the mix of communism and reparations, the economy crashes even earlier than OTL (1923), reparations are stalled, the French invade the Rhineland, the Germans form underground resistance groups. After a pretty expensive struggle (for both sides), the French pull out (1921), reparations are stopped, and the German economy slowly rebounds.
1921: The experiments with better results slowly pay off - Germany produces lots of fine products which sell well on the international market. Still, some of the other experiments keep causing trouble.
1922: The mechanisms to introduce reforms are modified to make sure that not every charismatic leader can introduce any crazy idea at any cost. Communism gets a "balance of power". This quickly leads to NEP-like reforms, but much more thorough, and longer lasting.
1923: A Central European Socialist Union is established, with Austria, Czechia, Yugoslavia, and Albania as members (Maybe a slightly different configuration - don't know which European countries might have become socialist or communist). Other countries join later. Capital is Munich. This expansion isn't liked by the western powers (including Italy), which causes some tension, but there's never enough support for war. A low intensity cold war is achieved by an agreement that Germany won't influence other countries to become socialist and/or join the "Munich Union".
1924: An economic boom occurs when finally the state power is accompanied by a functioning system of quality control, competence checks, competition, and so on, to increase efficiency. The whole country runs more or less like a single efficient company, with more or less independent private small businesses doing the outside work, while state run companies are doing the large mass production efforts. The boom is also fuelled by integrating the other countries of the Munich Union into the competition.
1925: German becomes official language in the whole Union, while other languages are still used as second or more official and first day-to-day language in the non-German areas. The German currency stabilises enough to be made convertible again.
1926: Democratic reforms reduce tensions with the West. Fascism, monarchy, and other "reactionary" forms of government stay illegal, though.
1927: The nationalist Polish military leader of the time (?) declares himself a devout socialist and applies to join the Munich Union, under the condition of some reforms, more autonomy of the regions, him keeping his post and some say in the Union, and first language status for the local languages in their respective areas. Germany accepts, as this also solves some problems with the own nationalists. The precedence makes a lot of other countries interested to join, and some other powers to form similar Unions - France manages closer ties with Belgium and Luxemburg, GB gets closer to Ireland again, and so on. An early version of the European Union develops in Central Europe.
1928: Further countries apply to join the Munich Union. No western power feels strong enough to do anything about it - especially as the joining countries join by their free will.
1929: Federations become the hype of the era, similar to fascism IOTL. The colonial powers try to reform along that model, with differing degrees of success.
1930: Similar to OTL, Germany becomes a leader in rocket and other research. It also already invents the computer thanks to lots of rationalizing everywhere and strong emphasis on science, even though not fully electronic. Thanks to a higher degree of autonomy and no reparations, Germany isn't hit by the world economic crisis very much.
1931: The remains of the Versailles treaty are ignored. Germany again mass produces tanks, ships, and other large weapon systems. But unlike the fascists and the SU, Germany doesn't build more than what would be normal for a country of the size and the economic development. Through the Munich Union, Germany already had a lot of tanks and other heavy weapons at disposal.
1932: The Munich Union is renamed Central European Union. Germany is split into Rhineland, North Germany, Bavaria, Central Germany, Saxonia, Silesia, Prussia, and Brandenburg. Finland joins. The Netherlands and Denmark agree on a common market. Tensions with Russia rise over a growing popularity of the CEU in the Baltics and the Ukraine.
1933: The CEU tries to find allies over the growing threat of the SU. Ironically, only Italy is willing, while France and Britain have their alliance, the US doesn't feel like forging alliances, and Japan is too far away ideologically to be a useful partner. Therefore, only massive arms production is seen as a possibility.
1934: Germanys arms production causes concern in other countries, an international arms race starts. The SU still produces more weapon systems, though.
1935: Italy does Ethiopia as IOTL. Germany cancels the alliance with Italy, loosing it's only partner.
1936: The Republicans win in Spain, unlike OTL. They have sympathies for Germany, but Germany doesn't want a confrontation with France and therefore avoids close ties. Still there is some military cooperation with and technological help for Spain.
1937: Japan attacks China (Manchuria) and is pretty successful, as IOTL. It too becomes a pariah state.
1938: The SU is busy helping the communists in China.
1939: The SU threatens the CEU over the Baltics. Japan attacks the SU over the support for the communist resistance, similar to OTL. The CEU aggressively supports independence movements in the Baltics and the Ukraine because of the aggressions. The SU, after having easily beaten Japan, decides on a surprise attack against the pretty much isolated CEU. The CEU is better prepared than expected (by both sides) and beats back the Russians.
1940: Both sides have turned into war economies. The CEU's technological superiority means deep incursions into the SU. Similar to OTL, Stalin transfers his factories behind the Ural mountains. Unlike OTL, Stalin gets little support from the West, and therefore has much more problems. The Ukraine, the Baltics, and White Russia are freed.
1941: Due to lack of support from the West, Russia looses Leningrad. CEU troops come close to Moscow.
1942: Stalin files for peace. The SU is reduced to Russia and Kasachstan, while the rest gets independent. The European parts of the former SU become part of the CEU. White Russia and a few southern satellites choose to join the rump-SU.
Task accomplished, even though there might be more twists and turns to it if Germany really had become socialist in 1918 or the likes. But the result might be similar.