This topic resonates with me.
I was brought up thru a relatively "rigid" style for 20 years thru the 80's under Akira Tohei Shihan who emphasized power development in his beginners. I still struggle myself with the transition to a more flowing style.
The statement that it differs from person to person is right on target, or as Tohei Shihan used to say, "case by case".
It seems that some folks need to get harder and some need to get softer.
Kisaburu Osawa, Hombu Dojo Cho in the seventies, said that he wished he had started training in Ki no nagare
much sooner because it was so much more difficult than the harder styles.
Myself, (being a hopeless recovering engineer) I refer to the 4 levels as:
1) Power = Physics
2) Technique = Bio-Physics
3) Harmony = Psycho-Physics, and
4) Michi = Meta-Physics.
Luckily for me, all the levels seem to present themselves all the time, no matter the focus. I have written an article on this very topic called Aikido for Recovering Engineers
which is a light-hearted look at the levels from a somewhat mechanistic viewpoint.