Hi, as far as I know,
prime-time drama ratings in Japan have sunk even lower than in South Korea these days. To get something that was really a big mainstream phenomenon (if any), I think you need to go back to the '90s.
Tokyo Love Story, any drama that
Takuya Kimura did in the '90s--I'm sure people more knowledgeable than me will give some recs.
If you're talking about
new dramas, however, I'd second what @AnanyaS had recommended:
We Married as a Job, which was really a major success and as close as a Japanese DotS that I can think of. Oh, and it even sparked a cultural phenomenon called the "Koi Dance" (look it up).
And since you mentioned about your grandmother and how you wanted to have something to talk about with her, what about trying some
asadora? They're morning daily dramas, about 15-minute per episode. (Quite popular among women, and traditionally almost always enjoy high ratings.) The episode count is rather intimidating--but hey, they're short, and since they're made so that homemakers can watch them while doing chores, you can skip and fast-forward if some parts felt boring, without losing the gist of the story.