[gambiting]

OP said:

"Indeed, that's how my own labour income is already taxed. I first pay income tax for 2021 based on my income in 2020"

It sounds like they pre-pay tax for 2021 based on what they paid in 2020. If that's not how it works then I retract my statement.

[bzbarsky]

That is a thing you can do in the US. It is not a thing you have to do, by any means.