Because my 2020 return was still "processing", the calculation was done based on 2019 income which was higher. How can I get the stimulus now that my 2020 return has processed with eligible income?
The IRS says in this document:
If a taxpayer's payment is less than the full amount and is based on their 2019 return, they may qualify for a supplemental payment after they file their 2020 return. The IRS will automatically reevaluate their eligibility. If the agency determines that they are entitled to a larger payment or the full payment, it will send them a supplemental payment covering the difference between what they originally received and the larger amount. If the re-evaluated amount is smaller, they won't need to pay back the difference. Aside from filing a 2020 tax return, no action is needed on their part.
If you do not receive it 2021, you may be able to claim it as the Recovery Rebate credit on your 2021 tax return.
The IRS says in this document:
If a taxpayer's payment is less than the full amount and is based on their 2019 return, they may qualify for a supplemental payment after they file their 2020 return. The IRS will automatically reevaluate their eligibility. If the agency determines that they are entitled to a larger payment or the full payment, it will send them a supplemental payment covering the difference between what they originally received and the larger amount. If the re-evaluated amount is smaller, they won't need to pay back the difference. Aside from filing a 2020 tax return, no action is needed on their part.
If you do not receive it 2021, you may be able to claim it as the Recovery Rebate credit on your 2021 tax return.
There is nothing you can do to get the third stimulus payment in advance if the IRS doesn't issue it after processing the 2020 return by themselves. All you can do is apply for the credit on the 2021 return you will file next year just like any missing first & second payments were applied for on the 2020 return.