Why exactly is a Hertzian dipole inefficient?
You haven't considered the loss in the matching network - practical inductors have a Q of only a few thousand, these would add significantly to the loss.

You haven't considered the skin effect - at 900 MHz only the outside few microns of the metal are effectively carrying any current.


The most efficient-for-their-size antennas are a bit like two hemispheres fed in the middle, so your idea is as good as it gets.
Edit to add:
Since matching networks have limited Q, the unmatched Q of the antenna is of little practical interest.
The main problem with high Q antennas and matching networks is that the available bandwidth is reduced, until they become useless. This is where there has been some interesting research.
Look up the Chu-Harrington Limit - relating the maximum efficiency, bandwidth and dimensions of electrically small antennas.   This paper has a graph.

I have in my head a more recent graph with the same limit line, but various newer antennas like the two-hemisphere and others, but I can't find it.