Is merely getting up for Suhur before Fajr with the intention of making up a fast (qada') sufficient as a nia (intention), even without uttering it or specifying it for qada'?

Intention (niyyah) is an act of the heart that does not require verbalization. So, if you intended to fast a make-up day (qada') before sleeping, this is sufficient, and you are not required to renew the intention or verbalize it at the time of suhoor. Making the intention overnight is a condition for obligatory fasting, and this occurs during the night, between sunset and dawn, based on the Prophet's (peace be upon him) saying: "Whoever does not firmly intend to fast before dawn, there is no fast for him."