The Democratic People's Republic of Tankiejerk

but doesn’t collaborating with the freikorps mean collaborating with what later became the nazi party?

The Freikorps weren't really the basis for the Nazi Party, which drew its support largely from the middle-class. The Freikorps were a bunch of separate independent paramilitary orgs filled with WW1 veterans who'd been officially thrown out of the German military by the terms of the Treaty of Versailles reducing the size of the German military, not any single, united group. The term is more for convenience.

The primary relation of the Freikorps with the Nazi Party was that the Nazi Party put great effort into courting disaffected WW1 veterans. The Freikorps themselves were effectively dissolved as the Reichswehr (the German military, still filled with old Imperial German officers, traditionally very 'independent' from the civilian government) became more closely aligned with the civilian Weimar government, and that dissolution (~1920) was, itself, long before the Nazis had become a major force in German politics (1923 at the earliest; more realistically 1928).

don’t get me wrong, i hate tankies as much as anybody should! i am just not sure whether the SPD way was more certain to succeed or better in general

I would say that SPD was better positioned to succeed and better than the KPD. That doesn't, however, mean that their hands were clean, or that their ideological approach was the best possible in the circumstances.

However, as the saying goes, you play with the hand you're dealt, not the one you want - the SPD was the best realistic choice available in Weimar Germany. And the SPD, until Thalmann came into power in the KPD, was willing to work with the KPD to present a united front against the German right-wing - even after a second attempt at couping the Weimar government in 1923. It was only after Thalmann took over the KPD in 1928 and obediently took Stalin's marching orders that the united front fell apart - not coincidentally, ushering in a period of increased right-wing electoral success and ending previously successful left-wing intiatives in Weimar Germany, culminating in the election of the far-right Nazi-Conservative coalition in 1933.