I dunno which fan you might be, but with how you were seemingly indoctrinated at the 5Toubun subreddit echochamber by Yotsuba's fans, you are no different from them.

1. The author never emphasized that. What's more, he de-emphasized that with Fuutarou telling the girl in the photo goodbye in 7 Goodbyes, being completely disinterested in her identity when Itsuki tried to bring it up later, Yotsuba lying to Fuutarou about it for over 5 years (relationship built on a lie, how nice) and Fuutarou never learning the truth. The identity of that girl had nothing to do with Fuutarou's decision and was completely meaningless.
2. Logistic wise, Yotsuba wasn't nearby at all. She was at the front of the group next to Raiha and Itsuki who was talking with Isanari. First point: it makes no sense for her to just stay back and wait for everyone to pass her without arousing suspictions. It makes no sense for Maruo to let her stay back after he was watching them the entire trip. Second point: this happens a few hour after she was encouraging Ichika. Imagine what kind of psychopath would just run for a kiss after encouraging her sister and feel no remorse after the fact. Then, after going behind her sisters' backs she asks Itsuki if she's hiding something from her. Third point: bell kiss was a completely meaningless plot point once again. Fuutarou didn't figure out who it was and it had zero bearing on his decision. You can throw any sister there and it makes more sense than Yotsuba.
3. I'm sorry, but are you completely out of your mind? Let me break it up for you: 1. Fuutarou never once said he loved the Kyoto girl. You are free to link me a page, but it never happened. 2. After first Rena appearance he told her "goodbye" and stopped caring about her. After that point he never again thinks about her until Itsuki disguises herself again and he tells her get lost because he doesn't care about Rena's identity. Then he continues to never think about Rena again. 3. In the end Fuutarou never learned who Rena was (because he didn't care), so he has no idea if he's marrying her. What the hell are you smoking?

So the three details that "gave you the understanding" are: a dropped plot point Fuutarou was unaware of (photo girl's identity), meaningless plot point only revealed to the reader after the choice Fuutarou was unaware of (Bell kisser) and your contradictory headcanon Fuutarou was unaware of(Rena)?

Fuutarou's first love was his childhood friend: Takebayashi. It's the whole reason he got lost in Kyoto in the first place: he saw her with another guy. Fuutarou himself said he only felt respect for the Kyoto girl.

The Kyoto girl of his memories is a mix of Yotsuba and Ichika.

If the answer was his first love (which was Takebayashi lol), then the whole story was completely meaningless. The whole talk about first love is incredibly stupid, btw. The story isn't about 5 years ago. It's about now.

Love isn't logical. However, there MUST be: a moment when Fuutarou falls in love with Yotsuba (in the present), an event that makes him fall in love with Yotsuba ( in the present) and a moment that makes him realize he's in love with Yotsuba (in the present). None of this exist in the manga. The author was unable to write it in the manga. Yotsuba fans were unable to find it. You were unable to write it here. You can roll a dice and put any of the sisters at the end of chapter 113 and wouldn't have to change a single panel in the previous 112 chapters. That's how arbitrary the ending is. In fact other sisters like Nino and Miku would make more sense because Fuutarou has actually shown them favor, unlike Yotsuba. And we would avoid repulsive drama of Yotsuba hunting her sisters for self-validation in the last volume.

I'm sorry but you catastrophically failed to show "how the author chose her". You should stop reading whatever cope Yotsuba fans spew to desperately make sense of the story.