The amount of EV credit that you get has nothing to do with your AGI, withholding, refund, or state tax.
The credit is applied to your tax liability before any nonrefundable credits, which is line 18 of Form 1040. All the nonrefundable credits, including the EV credit, are added together. The total is on Form 1040 line 21. On line 22, the total nonrefundable credits on line 21 are subtracted from the total tax liability on line 18, but the result cannot be less than zero. If the total nonrefundable credits on line 21 are more than the total tax liability on line 18, then you are not able to use all the nonrefundable credits, and line 22 will be zero. But it's not any one particular credit that is reduced. It's the total of all the credits that is limited. If Form 1040 line 22 is not zero, you are getting the full amount of all your nonrefundable credits.
Withholding is subtracted later, on Form 1040 line 25a. It does not affect the amount of nonrefundable credits that you can use on line 22.
