Presuming God’s forgiveness with regard to a venial sin would definitely be a sin, but considered in itself I don’t think it would be mortal. I am open to correction on this point. In any case, I don’t think all venial sins are committed with the idea that “God will overlook it.” Sometimes we know something deserves punishment, and we do it anyway. Sometimes we sin out of weakness or habit or for a variety of other reasons. It isn’t always with presumption.
Usually when we talk about the “sin of presumption,” we are referring to a vice which is opposed to the virtue of hope. Since hope is a disposition toward God concerning our eternal salvation, presumption would likewise be an attitude regarding eternal salvation, namely, that we can get into heaven without God’s help and no matter how we live, or at least that God will overlook mortal sins we have committed without any contrition or intention of repenting.