It is almost impossible to fly out of the void with an elytra without spamming fireworks because the firework pushing the player up is immediately deleted in the void. This means all fireworks (Duration 1 to 3) will have the same extremely short propulsion in the void.

Steps to Reproduce:
1) /effect give @s regeneration infinite 255
2) Give yourself a lot of fireworks with duration 3 and fall into the void
3) Try to fly back up
4) Compare how far the firework pushes you in the void and out of the void