You should wait to amend your return until the original return is fully processed. Your husband should file for an extension and pay what he thinks he will owe (if any) together when you amend your return to add him. If you will owe be certain to pay by 4/15/24.
You could prepare a mock return on your desktop version to calculate the total taxes due, then pay directly on the IRS website if needed. Don't forget your state taxes too.
File online extension here.
File state extension here.