I can't think of a single marking that will do this; you'll likely just want to have text clarifying what you want the performer(s) to do. Options like:
Vary the dynamics randomly while staying within the piano range.
But for what it's worth, I felt like your recording exceeded the piano dynamic range. Maybe something like:
Randomly change the dynamic level of each pitch, keeping the dynamics between pp and mp.
You may also want to consider the following addition
Randomly change the dynamic level and strength of articulation of each pitch, keeping the dynamics between pp and mp.
Plenty of other options exist, of course.
And I really like your description of "a resilient and ominous drone," but I'm not sure I would have read into it enough to play it as if it were "meant to appear to reduce itself only to keep coming back." If I were a performer, I'd be more than happy to see that indication written in, as well!