Retirement tax questions

The 8606 is what must be correct and filed with the IRS. For the 1040x, you only need to enter the amount of income change. For example: In 2020, $25,000 was taxable and the rest was basis. Then the amended would show only $25,000 subtracted from income - the actual income amount you were taxed on due to your basis.

Use the desktop software. The 1040x is a very straightforward form. I would keep it simple and override both the income and the 8606 in Forms mode for each year. You will want to file the corrected 8606 for each year.