It is too late to amend 2020 to MFS.  An amended return would have needed to be filed by deadline May 17th or Oct 15th if extension on file.   However, you can use 2020 to determine if 2021 could/should be MFS.   I have married couple that files MFS - each claims a child and each receives the child tax credit. Your calculations are correct.   

Was there any change in the RRC when you created MFS returns for 2020 ?