Can I base my 2021 Tax return on a 2020 amended return that has mailed but not processed?

[fanfare]

You must.

[munnik]

No need to file an extension and wait for the amended return to be processed?

[xmasbaby0]

No.   It will be many months before your amended return is processed.   You do not have to wait for that in order to file 2021.

[fanfare]

extension of time to file  is not an extension of time to pay the amount you owe.

If you expect a refund, after your 2020 1040-X is processed, there is no penalty for filing 2021 late.

It could be 2023 before you hear from the IRS about your amendment.