I'm inclined to follow Jim's approach here and just list out all of the possible emotional/relationship things we see and see where that gets us. (Not that I disagree with your initial assessment.)
Seok-Woo & Su-an: The biggest emotion that I got here was that he lets her down.
Seok-Woo & Sang-hwa: Seok-Woo lets him down too! Especially near the beginning
Seok-Woo & Seong-kyeong (his wife): He is tender/deferential to her needs
Many People & the Homeless Man: They disregard him
Jin-Hee & Yong-Guk: They grow into mutual, emotional support
I am having trouble pointing to many things, too. When he disappoints his daughter on the bed, his initial reaction seems to be disappointment.
When his mother tells him to talk to his wife in Busan, he quietly takes it on the chin (and tells hers it's taken care of) so she'll drop the conversation.
When he has the plan to go the Eastern Plaza to avoid quarantine, he tells his daughter think of herself first (an attitude) as opposed to running to tell the others.
It's hard for me to look past "something escaped from the lab" as the thing that kicks this all off. It's not like his decision to sell all the stocks caused this. (Unless I'm misunderstanding what happened at the beginning and what his subordinate Kim means when he says, "This isn't our fault, is it?")
Even the decision to go to Busan is because the gift was a bad gift.
The ending certainly seems like a decision, because they are weighing whether or not to shoot, but I think it's the action of singing that really clinches it. There is no decision to be made at that point.
In the middle, the decision to go from Car 9 to Car 15 is because their families are stuck in the middle. So I see the action driving that, too.
Of course, there is the decision to not let the two groups "merge" in car 15, and the old woman's decision to open the door... but do those change the perspective of the movie? I don't think so.