The Spartakusbund actually had a fit of bad luck IOTL. If they had managed to avoid the "January uprising" then the Freikorps wouldn't have been able to kill Luxemburg and Liebknecht, and they could've had a somewhat better outcome later in the year. If you really want to give them a better chance, though, you'd need to have Luxemburg and Liebknecht organizing a left wing within the party from before 1910, and be able to challenge Ebert for control of the SPD when Bebel dies in 1913. You could get a wholly different situation in WWI, no SPD voting war credits moment in 1914, and a much stronger revolutionary movement - the SPD would probably split but a much larger Luxemburgist wing.
A red Germany necessarily means a very different 20th century. The USSR proceeds differently and doesn't dominate the Comintern like IOTL. Revolts that IOTL fail may have better international support and succeed, like the biennio rosso in northern Italy in 1919-20, the Hungarian soviet republic of 1919, the Shanghai commune of 1927. Spain would turn out wholly different if Germany and all or part of Italy were communist by 1936 - not only would fascism not have its reputation but the main forces arming the Nationalists are gone, and the Republic gets German arms.
Of course, counter-revolution might also be emboldened, you could see a major backlash to communist Germany. But it would be very different with Germany under a red flag, you'd have actual shared prosperity and not the generalized misery of actual Soviet Russia that helped Stalin and the bureaucrats rise to power.