Hi, I'm commenting from America and my Japanese is bad, but I love your video! In my head the big difference between major and ionian is that ionian sticks pretty much to the same 7 notes even in the harmony, whereas major can have more freedom. At 3:21 you use a secondary dominant (E -> Am, or V/vi -> vi) which feels more like major than ionian because of the G#, ionian usually stays within the key to resolve in ways that aren't always leading tone -> tonic (if you want to go to vi maybe iii -> vi or V -> vi). This is the same way aeolian/natural minor (v -> i) is different from melodic minor (V -> i). Other than that I love your arrangements and all of the different styles!