The relative timing of the Roth conversion and the 401(k) rollover both in 2023 is irrelevant. The taxable result on Form 8606 is the same no matter which was done first. Having a nonzero balance in traditional IRAs makes your Roth conversion largely taxable with only a portion of your basis in nondeductible traditional IRA contributions permitted to be applied to your Roth conversion. The portion of your basis in nondeductible traditional IRA contributions that is not permitted to be applied to you Roth conversion remains in your traditional IRAs to be applied proportionately to future traditional IRA distributions.
