As @Malagachica pointed out that is the first and most important question you need to sort out.
Yes, there is a tax treaty between US and Euro BUT sadly it is very complicated and I have always felt we pay double taxes. The other thing you have to consider as a US citizen it doesn’t matter where you make $$ the US always has claim to it for tax purposes. You will need an accountant in Spain as you will have to pay taxes there and then you will need an accountant in the US who knows how to properly claim foreign income.
Creating a Spainish account doesnt matter, you are still on the hook for taxes. Also, Spanish (EU) banks are required to report any accounts held by Americans.
Since im dual and my husband is not US we technically could probably avoid paying US taxes on money we make in , but it just seems like a ‘why poke the bear’ situation.