Yes, it is suggested an amended return be filed after the original has processed.
Of course tax is due April 15th whether the original or amended return has been processed by the IRS.
With that said, the result of an amended return (Form 1041X) is always based as if the original refund or tax payment was completed.
If the original showed a refund, and the amended shows tax due, the tax due is after the refund is claimed.
There is always a chance that the IRS will find your error on your original return and hold up the original refund.
If that happens, you will get a letter from the IRS explaining the error they found, the resolution the IRS suggests, and asking if you agree.
Either way, you don't want to have tax due after April 15th if you can help it. Better to over-pay and request a refund later rather than be late paying. You could monitor with "Where's my refund" and make the payment the earlier of - once the refund is issued, or April 15th.
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