Although it is no longer possible to use the elytra and the mace's smash attack at the same time, you could still accumulate bonus mace damage while using the elytra. This allows you to utilize the elytra for extra control of movement, then swap it out at the last second to perform a smash attack, with damage that takes falling before dequipping the elytra into account.

To reproduce:

1. Using the following command, summon a ravager with no AI near the world spawn or a spawn point: 

    /summon minecraft:ravager ~ ~ ~ {NoAI:1}

2. Equip an elytra in the chest armor slot and a chestplate in the offhand.
3. Use the following command to teleport high up: 

    /tp @p ~ ~75 ~

4. Activate the elytra by pressing the space bar right when you start falling.
5. As you approach the ravager with elytra enabled, swap out the elytra for the chestplate by right clicking at the last second, then attack the ravager.
6. Notice that the smash attack takes into account the fall damage accumulated while using the elytra, rather than after it was dequipped in step 5.