@fanfare I appreciate the suggestion to efile and then mail in a 1040X with the manual 8606. But I was worried because the 8606 instructions say
File 2021 Form 8606 with your 2021
Form 1040, 1040-SR, or 1040-NR by
the due date, including extensions, of
your return.
And by your method the 8606 is not filed with the 1040.
But I found authority for either filing a 1040x as you suggest or a "second" 1040 before the due date. (this does NOT say extended-due date). Basically in that case the IRS considers the 1040x or second 1040 to be a "superseding return" and will deem it to be timely filed and (at least for a second 1040) to override anything on the first return.
See I.R.M. 21.6.7.4.10 https://www.irs.gov/irm/part21/irm_21-006-007r#idm140031113580544
Also of interest is https://www.meadowscollier.com/amended-tax-return-or-superseding-tax-return
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