I tend to think that the "Economic Impact Payment" would stay in English, perhaps with something like Corona-Hilfszahlung?
The Recovery rebate credit is the name of a specific Steuergutschein, so I think it could stay in English as well, at least at first mention,
Recovery means what you'd think, recovery as in economic recovery.
The rebate means that the credit is refundable - normally a tax credit in the US reduces any amount of taxes you taxes, but if you get down to zero taxes owed and still have credit left, that remainder will not be paid out to you. In this case the remainder will be paid out, so you could make money filing your taxes (albeit only if you were entitled to but didn't get the EIP)
Someone at the link above translated rebate credit as Rückvergütungsgutschrift, but that sounds kind of dodgy to me.
Edit, sorry, forgot the link. I didn't find the same one again, there are quite few sites that use this German word, I'll leave it up to the DMS to decide if it is ok.