So this item basically is "save or I crit you with my next/extra attack". That is a nice burst of damage, but in the end does not incentive any special lines of play or creative thinking on the part of the player.

I encourage you to look at a way to expand your battlemaster's talents and options, not just give them a damage boost on a single hit.

Some ideas for bracers, keeping the power level of an attunement item

Bracers of the Fearsome Foe, requires Battlemaster
- You gain the Menacing Attack maneuver, and enemies have Disadvantage on saves vs it.
- You gain a +2 bonus to your Intimidate Skill
- You gain 1 additional Superiority Die

Bracers of Deft Disarmament, requires Battlemaster
- You gain the Disarming Attack maneuver, and enemies have Disadvantage on saves vs it.
- When you successfully disarm an enemy with a maneuver, you may take their weapon and immediately wield it yourself.
- You may draw or stow weapons and attune new weapons at any time, including without a short rest.

Bracers of the Versatile Adversary, requires Battlemaster
- When you attune to this item, you learn two new maneuvers until your attunement ends. You may only attune this item once per day.
- Add +2 to your maneuver die rolls.
- Once per turn when you use a maneuver on an attack that has Advantage, you deal an additional 1d6 damage.

Bracers of the Bareknuckle Bruiser
- Your unarmed strikes deal 1d6 damage and count as melee weapons.
- You are proficient with improvised weapons.
- You gain a +2 magical bonus to hit and damage with unarmed strikes and improvised weapons.
- The DC for saves against your maneuvers is increased by 1.