You misunderstand how the credit is awarded. You need to look at your income tax liability, that is the amount the IRS keeps for the year, regardless of withholding. For example, if you have $10,000 of withholding and got a $2000 refund, your tax was $8000. If you have zero withholding and owe $8000 when you file, your tax is still $8000. Also, some taxes don't count, like self-employment tax, and the penalties for early withdrawals of IRAs and other qualified retirement plans, improper IRA and HSA contributions, and so on.
If you expect your tax liability (what the IRS keeps) will be $7500 or more, you would get the full benefit of the EV credit. If you change your withholding, you will get more money in your paychecks now and a smaller refund at tax season, but that doesn't change your eligibility for the credit.
