@saiyen
Now addressing your IRA's specifically, you wrote:
"To be specific, my pretax IRA accounts consists of:
Vanguard Rollover IRA (old 401k rollover to IRA)
Fidelity Traditional IRA (contains non deductible contributions for 2022 & 2023)"
If the Vanguard IRA only contains non-taxable 401k contributions and their earnings, you can rollover the entire Vanguard IRA to your current 401k.
Then, assuming your Fidelity IRA only contains non-deductible contributions, you can convert it to a Roth IRA any time you want. Since the conversion will happen in 2024, it will be reported on your 2024 tax return. Only the earnings will be taxable, not the original contributions. (But, a reminder that the non-deductible contribution for 2022 should have been reported on your 2022 tax return and generated a form 8606 in your name, and the 2023 non-deductible contribution will be reported on your 2023 tax return and generate a new form 8606 in your name.)
When you prepare your 2024 tax return, your 2023 form 8606 and the information about the rollover to 401k and the information about the conversion will all be reported in Turbotax, there will be no problem calculating your basis or determining the taxable portion of the Roth conversion.
Then additionally, you can make a new non-deductible contribution to the Trad IRA for 2024 and roll it over to the Roth IRA. You could do this later, separately; or do it now together, and it won't change the taxes. (In other words, you could convert the Fidelity IRA to a Roth in February, and then make a new contribution to a trad IRA in March, and convert that to a Roth in April; or you could make a 2024 non-deductible contribution to the Fidelity IRA tomorrow, and convert the entire amount to a Roth IRA next week. Either way, the taxes will be the same at the end of the year.)
Then for 2025 and beyond, repeat the process as I described for your spouse to make new backdoor Roth IRA contributions.
Just make sure that if you change jobs again, you either leave the money in the 401k, or roll it over into the next new company plan. If you take it out as an IRA, you go back to being unable to do a true backdoor Roth.
