A suggestion, allow Thrust with Firework Rocket.
Maybe it requires enabled on both server and client.

Juse a imagination:
Like 1 Firework Rocket = 100% thust for 1 seconds, or 10% thrust for 10 seconds. Or afterburner mode: 300% for 0.1 seconds
Maybe we can add something like fuel level, maximium volume is 1 Firework Rocket.
After the fuel runs out, it will refull with next Firework Rocket automatically. (And send a use Firework Rocket packet to server)
If there is no Firework Rocket on backpack, it goes into gliding.

While I would love to have this feature in the mod, I would say as the mod is fully client-side, all we could have now is automatic rockets use as 'vanilla thrusting'. You press w, rockets are activated, good.
also yeah it would be nice to have some GUI to see how much rockets left.
it would be such a banger