Weirdly enough, D&D has no rules for holding back when attacking someone. I've been making a strategist type of character and one of the things I wanted to be able to do is shoot an arrow our monk and have him redirect it, but his Deflect Missiles class feature is dependant on him being able to reduce the damage of the projectile to 0.

     If we were to implement a homebrew rule on reducing the damage of your own attack, how should we do it? Do you think "disadvantage" on the damage would be enough?

I see this under the same umbrella as "non-lethal attacks" house rules. If the intention was to reduce the damage, then I rule it as half damage rather than disadvantage on damage rolls. Reason being, it's quick and simple. With that said, I wouldn't rule that you'd be able to do this with every source of damage and depending on how "realistic" the player would want the attack to look, I might make them roll a performance check (might even have them use dex or str with their prof bonus, if they have, but that's if you're group varying modifier to skill checks).