(at first MC is considered special for being able to understand sacred beasts, only later everyone can understand them because of the deus ex machina of "I made it so they can understand me." Also, my original impression was that the sacred beasts were usually hidden in some sort of sub space, but later the MC has to physically hide them, so no?)

going to the Adventurer's Guild

Firstly, just because she suddenly remembered her past life doesn't equate to her immediately forgetting the etiquette of nobility. It's like as if those 8-years of her life was cast aside for her old one just to make a point that she was a reincarnation of an old woman. That isn't an excuse, I expected that she would be either using it to her own advantage or struggling between her past life's personality and her current one. Little to no background was really given about her personality during her current life before she remembered, pretty much it was glossed over.

Secondly, her powers which can be used as a means to the plot such as complicating matters, crisis, etc. could have been as said used but was cast aside in favour of what I've previously said. This also includes the things about the sacred beasts which are fundamentally and technically could have brought an interesting point into the story such as their powers and stuff but was cast aside for "ah! cuteness! of course cuteness! I'm not even going to question the fact that these guys can technically desiccate a whole country if I or them wanted but ahh! cute!" and no one is even questioning at the fact how she even got these deadly creatures who supposedly lived miles away or some shit.

Thirdly, the major thing that annoyed me on this is the fact that no one, I mean literally no one is questioning her. Like how the f*ck did this 8-year old suddenly knew how to make this outrageous (I say outrageous because note that she is in a different world that apparently has bland food with no advancement on the food department whatsoever till she came) food nor even question at the fact that her magical prowess exceed above others nor how their daughter's personality suddenly shifted into something else as if they didn't know how she had acted for 8 years prior to her remembering anything.

Lastly, Sei's situation could have been a far more interesting plot. It had a large potential and then suddenly gets forgotten along the way. It had potential such as the situation of what if's, drama, etc. you know the drill. The same could be said for the other character's, except for the prince. Motherf*cker is creepy, he's like 13? 14? And is crushing on a 8-9 year old child and is planning to get her just because he fell for a cute face and she can cook good food. Pretty f*cking shallow reason.