What is a single word to describe 'Extremely Delicate'?
A word that implies fragility while being more poetic, and which rhymes with mellifluous, is diaphanous:
[Merriam-Webster]
1 : characterized by such fineness of texture as to permit seeing through
// diaphanous fabrics
// a diaphanous curtain
2 : characterized by extreme delicacy of form : ETHEREAL
// painted diaphanous landscapes 3 : INSUBSTANTIAL, VAGUE
// had only a diaphanous hope of success
Using it in the passage from the question:
I hear your mellifluous footsteps in the diaphanous hallways of my self.
Ethereal would be a good choice.
American Heritage Dictionary
adj.
- Extremely light or delicate: "An ant lion ... is lovely, long-bodied like a damselfly, with a pair of ethereal wings" (Jennifer Ackerman).