>>279981571
The beginning of Chapter 120, where Nino (with a calm reaction) asks Yotsuba how long she's going to call him Uesugi-san, shows that first names aren't a big deal at all when it comes to their relationship. That alone shows that they wouldn't react that way to first names, and they've never done that in the series. And when it came to living together, they made a bigger deal. Their grinning like that must be a bigger deal than living together or first names, since sex is a bigger deal than the two of them.
Also, let's not forget the challenge between Yotsuba and Nino during the five-year gap, where Yotsuba had to prove to Nino that her love for Fuutarou was strong in order to get Nino to accept their relationship. And then after the time skip, Nino accepted their relationship, which shows the romance in their relationship, which is a bigger deal than a first name. Nino wouldn't react like that if it was just about the first name or living together instead of a bigger deal, since we already know her reaction to it.
Before Yotsuba could say "sleep" in her sentence, she paused because of their reaction and wondered what they were grinning about. Again, they first grinned like that when Yotsuba wanted to say "sleep." Did Nino's "Fuu-kun" get a reaction, but a big deal like their grin? No, not at all.
I don't think his three ideas for a sequel were meant as a joke since he had already done two of his three ideas.
It's nothing new to me that we won't reach an agreement; it's been this way for five years, after all.
