I’m very late. But you need to get the Conciliators in charge of the KPD. How to do this:
Embed a spy in the Comintern by July 1928
Use Paul Levi’s “Cooperation with the KPD” action by July 1928
Then, the Wittorf Affair will trigger in August 1928. Pick the third option to keep the information secret.
Make sure you took the Foreign Ministry in the May 1928 election. Once you draw that card, pick the Soviet option. Give them aid without conditions (not the Reichswehr one) then pick the option that says something about “Do the Soviets know about the Wittorf Affair? Do they know that we know?” Once you do that, the Soviets won’t defend Thalmann.
Sometime in the summer of 1929 (I believe August, but it might be July) the KPD Conference will trigger. As long as the Left faction is stronger than the Reformist faction within the SPD (you can do this through which rally option you pick, or party organizations, or questions of ideology, or advisors), there will be an option for the Conciliators to replace Thalmann. Pick that option.
In 1932, make sure Hindenburg gets reelected (just endorse him, ensure the Nazis haven’t surged. I like to do this via WTB.)
Then in 1934, when Hindenburg dies, the KPD will contemplate running Munzenberg. If you have adequate relations with the KPD (I believe they need to be warm or higher), and the Left faction is still stronger than the Reformist faction, you can endorse Munzenberg. Make sure SPD + KPD support is 45%-50%, because all the other candidates (except maybe Gessler?) will start consolidating around someone to stop Munzenberg.
Hope this helps!