She was healing for the most part in the beginning of the war. Once she was going to go into combat she was able to make the seal, storing chakra isn't much of hindrance when you are healing the wounded. You could basically say she wasnt supposed to be going all out until that moment.
Yes it was terribly developed, but that is Sakura and Kishi for you.
Now this is where I'd disagree, but more so on a personal level.
Based on her deployment and later seen moments in the war, she went as follows.
Started off in combat with Kakashi's division. Likely. as a combat medic (mixing fighting and healing as needed).
Later on before or during the night, Sakura transitions to a full blown medical position and is stationed in the medical Unit near the Second Division.
Finally, she moves with the survivors to help Naruto.
Throughout the two days, there hardly is a time to just store chakra. She is either in constant combat using chakra... or spending time healing the injured...using chakra.
Finally, the allied forces meet up with Naruto where they are focusing all of their efforts to the final fight.
The only conceivable time she could be building chakra is when she is on the full blown medical detachment. And even then only do when no one is allowed to move.
However, she arrives at the final battle just normal.
Her seal forms in the seconds after she finished dumping chakra into Naruto.
In conclusion, Sakura either just so happened to have paused her seal completion at 99.9% complete, using the time she ran towards enemies to unpause, or spent the entire war constantly dumping her chakra into people or attacks, and just happened to sideline the last whatever percent that coincidentally filled up at the point we all know.
There's no practical time for her to have been building up chakra unless she's both giving people chakra and putting it into her seal st the same time.
Which I personally feel is rather stupid. Why bother having her take two days slowly building such a huge battle boost when you could tap a shoulder and tell someone, "cover me for like 3 hours, and I'll suddenly 20fold my chakra levels and combat power."
But... as you noted, unfortunately Kishimoto and reasonable development just didn't happen for her. Thus, we're stuck where she completes her seal at a time period in which is ludicrously convenient, and begs the question as to why it didn't happen or be hinted at (other than having Tsunade as a master) sooner.
Only in Naruto's incompetent opinion.
Are you stupid?
If you would have bothered to pay attention since the start of Shippuden, you would notice that everything that Tsunade has done, Sakura has done as well. It's not hard to believe that Sakura would develop Byakugo, considering that Sakura has learned everything that involves healing and mitotic regeneration from Tsunade.
And your comparison is gay, and has nothing to do with what I just said.
The difference is the skill level. Sakura has done things Tsunade has shown sure, but not as well as Tsunade does them. For example, the healing of Shikamaru.
Sakura clearly showed she had the skill to help him (the guy was saved because of her effort). But Tsunade was able to effortlessly fix what Sakura was barely managing to maintain.
For Sakura to complete Tsunade's master medical technique with zero foreshadow is simply poor development.
It's something you can see her to easily learn, but not within the time frame (primarily due to the circumstance of a full blown war) to suddenly master without a droplet of suggestion she's been working on it.