Thank you for your answer and insights. In the mean time I find a solution for this. As Andrei ask, do you "send" invoices to UpWork or directly to clients? If you send to clients, how do you convince them to give you their details? For the time being, I invoice UpWork as a US company. But I pay the VAT for the commissions and so.
If I will be interested in the future into a collaboration with your accountant, I will let you know.