Rolling the 401(k) over to an IRA before the end of 2021 has made your Roth conversion mostly taxable since now only a small portion of your basis in nondeductible traditional IRA contributions can be applied to any particular distribution or conversion from your traditional IRAs. The rest of your basis remains in your traditional IRAs to be proportionately applied to future distributions or conversion. There is nothing you can do to change the taxable result on your 2021 tax return.
The relative timing of the conversion and the rollover does not matter. In the calculation of the amount of basis that applies to the conversion these transactions are treated as happening simultaneously on December 31.
