As others have pointed out, 1918/19 is a much better moment.
The reddest Germany could get in the 1930s without Hitler would be with really good charisma and a lot of luck somewhere between 20 and 25 % for the KPD and yet the same for the SPD, if austerity politicians like Brüning and others like him are seen as utterly economically discredited, and news about FDR's New Deal in the US begin to spread, so possibly rather later in the 1930s. But the problem is Moscow's doctrine telling them to stay away from coalitions with social democrats. The Komintern only adopted the Popular Front strategy after they saw what happened in Nazi Germany. Without Nazi takeover, communists may not even join coalitions in France or Spain, either. If Moscow orders to stay away, but an independent-minded German KPD leader enters a coalition with the SPD anyway, then a split is likely, which would cost them their (likely feeble) majority again.
So, long story short: probably no chance of communists in power in Germany if your PoD is "No Hitler".