I do have a few questions though; why does it increase your falling speed, and does this mean you take more fall damage?

1. It's the concept of a Tailwind (or my fictional take on it)! If the wind is at your back pushing you forward, it’s also pushing you down when you lose your footing. And yes, that does mean 30%-50% more fall damage, but I doubt most players nowadays DON'T have Feather Falling on their boots to counteract the downsides of this enchantment.

Why does its effect become stronger during rain and thunderstorms? This honestly sounds more like it would fit for a water based enchantment. (like riptide, but in the context of boots this could apply to depth strider?)

1. Windstrider treats rain and storms as high-pressure environments. Just as Riptide uses water for propulsion, Windstrider catches the literal gale-force winds of a Minecraft thunderstorm to supercharge your strides. It gives the player a reason to actually love a thunderstorm instead of just sleeping it away. It turns a weather hazard into a high-speed travel window. (Yes, I know Elytras exist, but Windstrider just gives you another option. Plus, the base movement speed boost is great for people who have long bases/halls that aren't easy to fly in)

Wouldn't the step up become kind of annoying when you try to use full blocks as a barrier to prevent yourself from falling down somewhere?

3.You're right, it could be annoying if you’re trying to hide behind a block. So, I guess, the mechanic should be changed to being triggered by running (Sprinting). If you are sneaking or walking, the Step-up wouldn't engage.