I totally agree on the build-up aspect. Totally. And there are moments when the build-up that existed were truly heart-breaking, in particular, the scene when Yotsuba was on the swing by herself.
I do strongly encourage a second read. The ending does feel like "let's push this beyond it's value" with the five views of the last school event. But if you forgive the authors, then that part isn't so bad either.

I wanted to make an annoying distinction between "Character" and "Personality." Skip this part if you're already aware, but it took me some time to distinguish them.
Personality is that part of a person we "see" when we're with them: are they vibrant? seclusive? life of the party? cheerful? gloomy?

Character is the deeper part of a person that isn't so much easily on display: patience, thinking of others, and pulling them into activities (much like Tsubame-senpai in Love Is War is the kind of sweet, helpful gal--on the look out for any loners); giving, punctual... these are traits I look for in "a good story." So, when I see an MC end up with a person who is poor in terms of character, I think "Meh; lame."

The stories I find most enjoyable are where there's a moral to the story: "Get over your darn shyness and tell this gorgeous gal you love her", not just a satisfying end: "He got the girl!" For this reason my enjoyment of The Quintessential Quintuplets was not disappointing.

Oh, by the way: here's a breadcrumb you may have missed! Scene: MC is in bed with a cold at the end of the festival where the Dance is happening outside and all five sisters are holding Futarou's hand as he is in a deep deep sleep— Each of the sisters is holding one of Futarou's fingers (five girls, five fingers, amirite?) Yotsuba, yeah, pour on the cringe, is holding Futarou's ring finger. See what I mean by spotting breadcrumbs throughout the story on re-read?

Best Feature: Watching Futarou emerge from his sense of worth because of his amazing grades as Yotsuba emerges from her sense of worthlessness because of her lack of grades.

I simply grew to enjoy Yotsuba. Until they began to give away her past, I found her to be an enigma. When they revealed she was the girl in the photo, my heart ached for her, and I waited in hope that her love for Futarou would bear its patient fruit. The other sisters, I felt, grew to be nothing more than noise. And when Futarou confessed his love after having flopped onto the grass, I couldn't hold back the tears. And when he proposed at the playground, I couldn't hold back the excitement.

Final note: In the Anime, the campfire dance's "spell" was that whoever one held the hand of would be with the other for life, but the Manga says "Married to." Add to this: whoever kisses under teh hot springs bell will marry the one they kiss. So, Futarou is, in a way, "stuck" with all five sisters. He can't shake the four regardless. They'll all four be with him and Yotsuba till they die. But the one he marries is, as per the legendary bell's warning, married -- which were both fulfilled. Goofy? Ohhhhhh fer sure! Sweet, entertaining, romantic? Ohhh yush!

I was looking more along the lines of “who had the most clues/screentime given?” than “who is the best match for MC?” On a first readthrough, it seemed to me that Yotsuba showed almost zero affection towards MC untill near the end of the story (as you wrote, this is probably because I didn’t pick up the hints, but I knew beforhand that Yotsuba would be chosen at the end (read spoilers some time ago thinking I wouldn’t pick up the manga), which made this even more offputting). I genuinely thought it would be one of three; Miku, Nino, and Itsuki, in that order of likelihood.

Miku is the character imo that had the most character development in this manga, i.e. her personality changed on the inside. As the story went on and she interacted with the MC, she shed some of that emotional baggage. A shy girl opening up after meeting a guy? We see this all the time! I fell for it, Negi you motherfucker.

Nino was definitely an asshole at first, but she was the first to openly display her affection, which made her one of the top runners in my mind. Itsuki didn't have much going for her, but she was the very first character to have an interaction with MC, and that is sometimes a clue to who MC will end up with in harem stories.

I don't believe Yotsuba is a bad fit for MC. However, if the author intended to make the Yotsuba ending, I think it would have been possible to have some more buildup. You know, some affection shown. The premise was that Yotsuba was holding back because of her guilt towards the other sisters, but show something after that guilt has been lifted, yo.

But you might be right that Yotsuba is the right choice, just not the “safe” choice. Maybe I’ll give this piece another read someday.

I went through this story examining each of the quintuplet's "characters." I added their "personalities" second.
Ichika (the eldest): Like the others, she "tried." No question there. But she also betrayed a dishonest heart. I suppose that was meant to be displayed in the fact that she is an actress, a paid liar, so to speak. While she was the one I was rooting for, in the end I realized that the MC would reject her over her being simply too complex. He's a simple guy, and that aspect of him needed to be taken into account, as I understood it. Had her arc been more one of redemption, where she would have abandoned her movie actress desires along with her desires to be dishonest, I think she would have had a greater chance. Ironically, the MC has far too much difficulty expressing the truth (for my liking). So two joined liars would have been a disaster.

Nino (the second): What can I say? She did try, but it felt like "too little, too late." She overdid things when she disliked him. Likewise when she liked him. This girl was very persuasive, nevertheless. He was probably teetering on the edge of accepting her, had he been less wimpy and dishonest with himself. Character-wise, this gal soon became the top contender in my mind. However, her open expression of "I want you!" was a flag that this would not end well. I think Futarou just felt that she was too unpredictable. And she sank her own ship by pulling/pushing. Keeping in mind that if she were rejected, he had four copies of her as an option.

Miko (the third): And one of the fan favorites. Yeah, who doesn't want to vote for a shy, self-effacing girl who lacks confidence! Woooot for the underdog. But this is a bit of an armchair sports enthusiast's point of view. I never EVER saw this girl as having a chance. The stench of "LOSER" was all over her. MC's dealing with his own issues, not to have some emotionally messed up chick's baggage thrown on top of his for a major dumpster fire. I don't hate her. She's the fan favorite for a reason. And I believe she was tossed into the mix to deliberately throw the readership "off the trail." A "Miko" may make a story a fun read. But in real life, they're nothing more than dead weight to a guy's life. I wasn't even slightly persuaded from the first few chapters, that she had a snowball's chance.

Itsuki (the fifth): Itsuki's issues also ran too deep. They lay deep in her mother/self turmoil. This is important due to the differences between her and Yotsuba. Itsuki's issues were so deep that she was the most blind to her own feelings and to Futarou. Though she was the first to "date" him, indirectly, I just knew she was like the last planet in the solar system. As the story progressed, her coldness toward MC just help to seal this coffin even tighter. As a character, she seemed the least interesting. She had great potential, but the author kinda kept this sister "in the background" by being much less interesting than the others. She wasn't going to bring much to MC's party.

Yotsuba (the fourth): Upon re-reading the story, it's amazing to see just how many breadcrumbs were sprinkled on the path that led to Yotsuba's front door (and this is the kind of story that deserves a second read.) Though Yotsuba had her issues, there were not based in family, as much as her own disappointment with her inability to keep her promises to the MC. She's not smart, and she knows it. This led to a self-effacing attitude that really doesn't constantly taint her. She's excitable, fun, sweet, thoughtful, caring, and treats MC with the greatest of respect and kindness. What threw me off was her utter lack of "displayed" interest. As the story unfolded, it became obvious it was all a front: tied to her wanting MC to have the best of her sisters. The scene I found the most heart-warming is when she gave the MC a lap pillow and gently confessed that she liked him.

There is a glut of school romance manga/anime out there. Often the writers take the "safest" route and put 99% of the typical story elements into their tale.
However, this story stands head-and-shoulders above so many and remains 1 of my top 2, the other being Love Is War.
Personally, I cannot see anyone other than helpful Yotsuba to be paired up with helpful Futarou. She had the most positive character elements to bring to a relationship. Her problems were nowhere as deep as her sisters'.

So, yeah; I'll get a lot of hate on this one. But having dated some Miku-types in my past, I'm glad my wife is more like Yotsuba than Miko.

Now I know these kinds of manga cannot have everyone satisfied with the ending, but Yotsuba, really? There was just too little given away in the story to make her the convincing winner. Being the kid MC met years ago doesn’t cut it. At least Nino and Miku tried. I would have been fine with anyone but Yotsuba tbh.