Idk if I would give blame to the SPD for the actions of a joint coalition when they’re in the minority. Maybe I should?

I mean, at the very least they were enablers. The SPD at the very least voted for the bad ideas of the Union in parliament. But two things stick out: the SPD-Greens coalition 2002-2005 introduced Arbeitslosengeld II (commonly known as Hartz IV), a law that amplified impoverishment and precariousness of the lower classes, and the decision of the SPD after the 2013 election to enter a coalition with the Union instead of Greens and Left.

While they were leaders of their party, even despite the backstabbing FDP they passed several things that seemed good. For a moment it felt like Germany was moving in the right direction.

Yes, two of the three Traffic Light years were very hopeful. But the SPD also is not innocent of this coalition’s downfall. The FDP started that backstabbing and sabotage very early on and instead of showing them their place during a time the FDP would not risk losing their position of power, Chancellor Scholz faltered and caved in to tgeir demands, paving the way for FDP and Lindner’s later actions.

The SPD is spineless and rarely actually makes politics for worker’s benefits. Instead, the claw to power as much as they can.

All of that being said, my advice remains vote for Die Linke and Grüne first and foremost, and then if you have to the SPD

You don’t have to. Germany employs proportional representation for parties, meaning the parties get seats in relation to how many votes they get. The only exception might be the election of direct candidates, but that is a whole other discussion and not as attached to party elections as it might seem to superficially.