As a seasoned Minecraft player, I was thinking that the Elytra's flight behavior should be configurable. That's why, I created this mod.
It adds options to limit the Elytra speed as well as changing firework boost strength and duration. For the flight speed limit, there are two ways of calculating and applying it: horizontal, x and z axes only, or absolute all axes. The latter restricts vertical gliding too, so downward elytra flight is also capped. All this can be easily configured inside the configuration file.
Furthermore, it changes behavior both on the server and client, due to Minecraft calculating Elytra flight server-side, while the client predicts motion locally. Therefore, this mod changes how the server applies Elytra physics so it's also enforced even for clients without the mod. And it matches the client's predictions based on the server's configuration to enable smoother flight. Otherwise, the player may experience visual stutter or lagging back.
All in all, it's a flexible and lightweight solution with the ability to nerf or buff Elytra flight, that's up to your choice!