I honestly wouldn’t say they are keen to cooperate with the AfD, especially as long as it is possible to achieve their goals with a partner as malleable as the SPD

And when they don’t get what they want from any of the other parties they will cooperate with the AfD.
As they have done in Thuringia, as they have done in the Bundestag. Once voting together has been established as normal, forming a coalition is not a big next step.