We don't have a "voter will" or "Regierungsauftrag" in our Grundgesetz. This article is a good reminder of it https://www.freitag.de/autoren/jaugstein/kanzler-oder-ampelmaennchen (Just use DeepL if you want in English)
Besides the SPD is only ahead with 1,6% and had the 3rd worst turn out in history. It is impossible to say it is clear cut they should supply the chancellor.
Paul? Paul Ziemiak? Is that you? Can't fool me. It's you!
You don't need to have something in the Grundgesetzt. Looking at every post-election survey about who should be chancellor and what coalition is being wanted clearly shows what the german people want.
That is the "Wählerwille".
The CDU dropping almost 9% to their worst result ever is the same. That the SPD has it's third worst result doesn't matter, they had growth and ended up as the winner of the election.
That's "Wählerwille" here too, because the people voted to have the CDU drop like a rock and have the SPD as the winner.
Also, ultimately, what does it matter by how much the SPD won? Since when do you have to win by x% margin to have a "clear cut"? To use your own argument, "it's not in the Grundgesetz" how big the margin has to be. ;]
The CDU clowns, sorry, heads would be yelling from all angles about the "Wählerwille" and "winning the election" and thus having the "right" to fill the chancellor position if they'd won by even less than 1%.