@McLXIX I agree, Miku's character's developments feel a bit... sluggish, perhaps the author could have negociated them better. But I do not personally see them getting out of the scope of what a teenage girl, having finally made up her mind, could endeavor to become to win the affection of her love interest.
That said I am relieved to find a sensible reader in you, godspeed.
Itsuki gets a lot of shit by virtue of being main girl alone but she is actually quality girl content. People are so opposed to a main girl win that a main girl win would be the new side girl win.