An interesting thing I have noticed about the Summon Steed glyph, is that it always summons a horse with the exact same stats.
These stats are fairly average, consisting of 25 hearts, a jump height of 2.89 blocks, and a speed of 9.49 blocks per second.

This is not the best that a horse can be.
My suggestion for how Summon Steed could be amplified is fairly simple. It summons a better horse, with better stats. Potentially also wearing horse armor?