I agree about work, just disagree when it comes to money. What did Seju always done with Sumin? She bought her stuff, even if she never wanted it. And those weren't even her hard worked money. So even though the advice is partially right (come on, Sumin needs a job), I find her also wrong ;)
Now, you bring this up also @@. Seju manages a freaking company @@, how the hell is that not even her hard working money =))))). She didnt even use her father's company, and looking at the way of her latest family gatherings, while her brothers continuously failed in business attempts with their father's money, Seju seems to be successful, she even lends her brother money =))))) and even imply to be richer than them =)))))))
You are over-analyzing that sentence, it's just Seju's logic, it's not even an advice to tell Sumin to get a job =))))). It just means she thinks she rather work to make money =)))))
And this scene is no relation with the whole buying Sumin stuff. Why brings this up, I have no idea but come on, what's wrong with buying the person you love stuff? When you have the money? When you think it's more practical to buy it, instead of making it? Everyone has their own ways of expressing their love, why do you have to force to think this way is better than the other way?
I agree that it shows differences between SejuxSumin but other things? No, nope,