Is FDP joining a coalition with SPD something they're open to or what happens if they refuse?
They're open to it & will enter into negotiations. If they refuse, then probably Jamaica &/or GroKo negotiations.
Germany is in this weird situation where the victorious party lacks options and can't force their allies hands, and their potential allies are not all set on supporting the SPD. R2G is in the dumpster, and GroKo v2 has been refused by the Union. The partners, FDP and the Greens, have more leverage since they could walk away from the SPD and prop up the union, despite how it would bend a bunch of German political traditions.
There's little appetite among Green voters to work with Union, though. I don't see how Laschet has a path forward with them.