An updated version of https://www.planetminecraft.com/data-pack/nfinite-fireworks/
If you have a nether star in your inventory you will given a firework in the first available slot in your inventory. It stops giving you fireworks if you already have one so your inventory doesn't get flooded, but since it constantly gives fireworks you can hold right click to keep boosting with an elytra
Changes from the original: Nether star requirement (can be easily removed by modifying the pack, the source code is one line) Multiplayer support