Note that you always could have filed MFJ for 2020 and 2021, you would just have had to apply for an ITIN if you were not eligible for an SSN.
You can file amended returns to change to joint filing status for 2020 and 2021. It will not change the amount of child tax credit your spouse already received. It may lower your tax in other ways, since MFJ has lower tax rates and some deductions are more favorable. However, any tax benefits that would have required you to have an SSN won't be paid if you amend, because you needed to have the SSN before the original filing deadlines. (For example, you won't be eligible for EIC, since that would have required a joint return with 2 valid SSNs to be filed originally.)
