Bonus action- the victim makes a dexterity saving throw and has disadvantage on attacks their next turn if they fail. I love some of the more complex ideas here, but most tables will likely prefer something simple that won’t bog down the game.

This is just a fancy Help Action.

Yeah, I'm in the Help camp. We all love to design complicated mechanics, but complexity does not always equal depth. You can do your contested saving throws and applications of cover and then the next player is going to want to put a bucket over the enemy's head and will expect you to design a mechanic for that too. Just call it Help and focus on the narrative of the combat rather than slowing things down with unneeded mechanics.

I think I'd handle throwing sand similar to a bag of ball bearings or caltrops where it has a fixed Dex save DC around 8-10 and requires an attack against the creature that are of equal or smaller size than you which allows a bonus action attack with a light/finesse weapon. The effect would last until the end of the creatures next turn for simplicity's sake. This allows the player get the satisfaction of using alot without allowing it to become a disruptive effect in combat. This effect wouldn't effect creatures with more than two eyes.

The reason I'd make it a relative size effect is that a larger creature's eyes may not be in range, though for something like a beholder, I might make an exception as things could get interesting.

Slight of Hands vs Insight maybe?