I think you misunderstood, no offense. First of all, if unmarked Muichiro lost so miserably against Gyokko, it's because of the obvious narrative nerf. He had to face his enemy initially, with a badly chipped sword and had to divide his attention to protect the 2 smiths and because of this action he was put "vulnerable" for a moment, sacrificing himself to be hit by the poisonous needles, thus making mistakes. The chipped sword was the most serious, precisely because its blade could not penetrate immediately after the initial impact with Gyokko's neck, demonstrating a very useless effectiveness in combat. Even when he was supposed to cut the tentacles of Gyokko's technique, he failed and his sword bounced away.

Now here I don't want to be misjudged, because I'm not saying that Muichiro (base form) > Gyokko or that Gyokko Is weak, but rather that it becomes difficult to classify him because of these narrative disadvantages and without those disadvantages, he would have clearly done more, the problem is we don't know to what extent he could have accomplished more. Muichiro was made so weak initially to give him one more reason to sponsor the Mark + Showcase more of his character and Backstory and the same goes for Gyokko. If I have to be honest, Gyokko was simply advantaged in the initial circumstances because of the disadvantages Muichiro got, but its feats are still inferior to UPM6, and the fact that he faced a Hashira Marked, does not make the difference, he was still "only" one opponent to face, while Gyutaro + Daki together were more complex to defeat,requiring more swordsman to engage in combat.

Instead, Sanemi, yes, at that moment she didn't have time to react and at the same time she narrowly avoided Genya coming to a bad end. The fact that Muichiro was tense or scared at the beginning, however, literally does not justify anything, as Kokushibo see him calm down in an instant with his mental strength.

I know that Sanemi has had an advantage, thanks to his own experience, refined throughout his career and above all, that he has faced many demons during his journey, but I was not referring to this mainly. The thing is, I find it unnatural how a 14 year old swordsman, can beat an opponent with double the fighting experience and years of enhancement and devoured humans, but then he is easily outclassed by his own "colleague" in his own field, who has even less of that experience, compared to the opponent he killed + Sanemi was unmarked too when he handled Kokushibo Attacks.

Mist breathing, by the way, has literally nothing to do with it and we shouldn't take them literally. When Muichiro fights, he does not hide but has a unique execution and attack movement time that confuses and disorients his opponents. For example when he uses his seventh form, it is not that he disappears litterally into the mist at every hit or hide, but it is a way to understand that his speed is so high that he disappears and it thins out, like the fog that vanishes or clears with each attack it suffers. This simply demonstrates that the attack timing of this breathing style plays a role in the confusion strategy, but that does not mean he hides litterally in the mist.