How to reduce the height of a monopole antenna without using any lumped elements?

You can use a helix structure at the bottom. For a shorter antenna it will increase the inductance. Your picture is not visible.

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Only really two solutions.
A capacitor "top hat"

Or the whole thing can be made a helix all the way up.
Just wind roughly a quarter wavelength of wire all the way along a fibreglass fishing rod.

you can bent the monopole structure