Search Youtube for "elytra bounce" or "e-bounce".
Module automatically executes tick-perfect jumps as soon as you touch the ground after flying with elytra.

Module can be only activated from scripting.
If you want toggle key, use Key Bindings script like this: If you want hold key, use Events Scripting script like this:

Settings

First Jump Ticks:
Between Jumps Ticks:
Second Jump Ticks: