Fidelity was correct that the rollover from the 401(k) was not itself taxable. Fidelity has no responsibility to determine if having a nonzero balance in traditional IRAs has any effects on the taxability of a traditional IRA distribution. Saying anything with regard to that would be considered tax advice and for tax advice they instruct you to consult with a tax professional (your CPA) because Fidelity does not give tax advice.
There is nothing that you can do to change the taxability of the 2023 Roth conversion. The result is that only a portion of your Roth conversion is nontaxable an a significant amount of your basis in nondeductible traditional IRA contributions remains in your traditional IRAs to be applied proportionately to future distributions, so most of the benefit of having made the nondeductible traditional IRA contribution will be realized in the future.
If you have a 401(k) plan at your current employer, perhaps in 2024 you can roll over to that plan whatever amount in your traditional IRAs exceeds your remaining basis in nondeductible traditional IRA contributions so that you can that convert the rest to Roth tax free.
