@saiyen
"Is there a difference between converting into a Roth for 2024 vs 2023? My wife has already made her nondeductible IRA contribution for tax year 2024, I have not."
Let me address your spouse separately. A "backdoor Roth" has two steps. First, a non-deductible contribution to a traditional IRA. Then second, a conversion of the IRA to a Roth IRA. The contributions can be made any time. Conversions are not retroactive, they happen on the date they happen. Since we are in 2024 now, any conversion she does will be reported in 2024.
Taking your original statement: "My wife's pretax IRA accounts are just a Vanguard Traditional IRA with non deductible contributions from 2022, 2023, 2024".
Your wife can do a traditional IRA to Roth IRA conversion at any time. If the contributions for 2022, 2023 and 2024 were all non-deductible, then the only taxable portion of the conversion will be any earnings or growth in the account. The 2022 non-deductible contribution should have been reported on your 2022 tax return and generated a form 8606 in your wife's name (which is separate from any form 8606 in your name). The 2023 non-deductible contribution should be reported on your 2023 tax return that you are working on now, and will generate a new form 8606 in your wife's name. Any conversion she does will happen in 2024, so the non-deductible contribution for 2024 and the conversion will both be reported on her 2024 tax return, and she will pay income tax on the earnings portion of the conversion (and this will also create a new form 8606 for her).
Once that is done, then for 2025 and later, she can contribute up to the annual limit as a non-deductible contribution to a Trad IRA, wait a few days for the transaction to settle, then do the rollover to her Roth IRA. Assuming the investment is kept in cash for those few days in the traditional IRA, the only taxable event might be a few dollars of interest.
