I agree with Cubic's answer that you are adding an unnecessary mechanic. Behaviors like this are assumed to be part of your character's actions without being explicitly called out in the rules.

However, if you feel the need to confer some advantage for crouching in your game, you need to balance it with non-trivial costs. The only cost you have is that the character's effective movement rate is halved while crouching, but there is no cost to enter or exit the crouch, so the character can stand up, move, and crouch again without incurring any costs.

Ideas to fix this:

1. Make crouching and standing up cost something. Maybe it's 5-10ft of movement to do so, maybe its a bonus action. Paying a bonus action is more expensive than movement for some character classes, less for others, consider that when making your decisions.
2. Give some alternative penalty for being crouched, like disadvantage on dex saving throws for the reduced mobility. This is similar to the advantage granted to melee attackers against prone targets.
3. Make this a feat that modifies the Prone condition, eliminating the disadvantage for attacks made while prone (similar to the existing Crossbow Expert feat), maybe making standing up easier too (but not free!). All of the other mechanics of the Prone condition stay intact, with all of their associated costs.

Some combination of #1 and #2 would make crouching comparable to, but not exactly like, being prone. #3 makes this a special ability that few characters will have, and is roughly in line with the power level of feats.

I agree with Cubic's answer that you are adding an unnecessary mechanic. Behaviors like this are assumed to be part of your character's actions without being explicitly called out in the rules.

However, if you feel the need to confer some advantage for crouching in your game, you need to balance it with non-trivial costs. The only cost you have is that the character's effective movement rate is halved while crouching, but there is no cost to enter or exit the crouch, so the character can stand up, move, and crouch again without incurring any costs.

Ideas to fix this:

1. Make crouching and standing up cost something. Maybe it's 5-10ft of movement to do so, maybe its a bonus action. Paying a bonus action is more expensive than movement for some character classes, less for others, consider that when making your decisions.
2. Give some alternative penalty for being crouched, like disadvantage on dex saving throws for the reduced mobility. This is similar to the advantage granted to melee attackers against prone targets.
3. Make this a feat that modifies the Prone condition, eliminating the disadvantage for attacks made while prone (similar to the existing Crossbow Expert feat), maybe making standing up easier too (but not free!). All of the other mechanics of the Prone condition stay intact, with all of their associated costs.

Some combination of #1 and #2 would make crouching comparable to, but not exactly like, being prone. #3 makes this a special ability that few characters will have, and is roughly in line with the power level of feats.