Very rewarding to finally get an SPD majority on hard! You can actually get the Weimar coalition before September 1929 and it allows you to sidestep the right's vote of no confidence (well, most of the time anyway). Then all you have to do is implement the WTB and you're good to go.
Viewing post in Social Democracy: An Alternate History comments
Its basically just luck of the cards for the 1928 election. I found some combination of funding radio, organizing women with Juchacz, choosing all extremist parties as your enemies, champagning with urban workers, and rally on various issues can get you up to the 34% that you need to get the Weimar coalition.
As for 1932, if Zentrum wont coalition with you then having Otto Braun as an advisor can help. He has "Negotiating with the Coalition" which improves relations with them. It also helps not to try to associate with the KPD, though you can do both. Obviously there is the Inter-Party Relationships card, but that's more useful for the KPD than anything else. I usually don't have much of a problem with them even when I'm passing hella welfare and raising taxes out the wazoo.
I've tried doing this several times, but haven't gotten it, not sure if the order I did it was wrong or something.
That being said I keep on getting 33%, so my trick is to just make Müller chancellor, call a snap election as soon as I form a Grand Coalition, after which I campaign 2-3 times, then I can form the Weimar Coalition.