Since we are talking about One For All, it isn't really a rhythm game, so the only thing that's different between songs is the speed, the BPM (beats per minute). I'm not sure about most of the songs you list as easy, but i definitely remember Masterpiece being one of the slowest, if not the slowest song in the game, which makes it the best song for scoring later on. If it is indeed the speed/BPM that determine whether you think a song is easy or difficult, the BPM are listed in the song details when you are selecting a song iirc (they are definitely somewhere listed in the game). Just use songs with lower BPM, although iirc if it gets too low, there will actually be several button presses for each beat, but i think i encountered only one song like that in the game and i don't remember which one it was.