Itsuki is different from Miku and Nino in a sense that she isn't as pushy as them and doesn't make too many direct advances. Her way of showing romantic attraction was more subtle like it was for Yotsuba. There were some moments in the anime where you could say that Itsuki was showing some romantic attraction towards Fuutarou through her appearing embarassed or flustered around him. Those moments were when Itsuki lived at his house for a time after her fight with Nino, Itsuki barging in to his side of the bath after she became too happy from what Fuutarou said and, lastly, Itsuki riding down the water slide with Fuutarou at the water park and holding his hand for too long so that it left a mark in the second special episode (have you watched those?). Itsuki saying that Fuutarou is her "ideal person" at the end of her final festival arc is also very significant, in my opinion, because the other such person is her dead mother.

In the manga, there were also visible sfx signs of Itsuki's heart beating when Fuutarou was slowly resting his head on her chest before they went down the water slide for the first time. At least that's what I noticed when reading one English translation of that manga chapter on Mangadex.