>Frieren is deep
No, it's not. It's your basic "appreciate the little things and your friends while you can"
>Death Note is deep
No, it's not. It's your basic "playing God is wrong"
A truly deep manga/anime makes you go beyond the basics, beyond the surface, beyond the obvious.
So my question is: has any work ever done that to you?
99% of so-called deep manga/anime aren't deep at all.