I think Ian and Mason are both really great boy names. But I voted Cole for two reasons. One, I think it goes well with your dd's name. Two, I think a one syllable first name flows nicer with a two syllable last name. Especially since Ian and Mason both have the same number of syllables AND end the same as your last name. I like the first and last names to end differently - just my weird naming thing.
I think you could swap out the middle names, depending on your preference of which you'd rather use more. I love middle names after people you love.
And our dd1 is MacKenzie Jean.
I didn't want the "a" but dh did....