STOP ... amend the 2019 first if you have something to carry forward to the 2020 return (even if you will file the 2020 return first) so the carryforward will be correct. If you don't then the 2020 return will also need to be amended ... a waste of time and effort.
Once the ITIN is issued mail in the amended return ... it will take months to process but the 2020 return will as well so don't expect a quick turn around on either return.
Another FYI ... you may be able to efile the 2021 return but you will need to use ZERO for the 2020 AGI since the 2020 return is being filed late.
