The only things needed for the missed RMD years is the Form 5329 for each of these years requesting waiver of the excess accumulation penalties. Nothing changes on the filed tax returns. You should be able to file these without including Form 1040X, but you'll need to include an explanation statement with each. Simplest will be to prepare the Forms 5329 manually using prior-year forms and instructions downloaded from the IRS. The instructions for requesting a Waiver of tax are on the last page of the instructions. When filing Forms 5329 stand-alone, be sure to sign and date each of these forms.
https://apps.irs.gov/app/picklist/list/priorFormPublication.html
Prior to filing these Forms 5329 your wife should receive a distribution of the total of late RMDs (and perhaps also including the 2019 RMD) so that in the explanation statement you can indicate that she has taken the necessary action to make up the RMD shortfall. This distribution will be reportable on her (your joint) 2019 tax return.
