I've decided to choose Belgium for the Euros... but I finally got this answer from Wise today.
"Wise uses local banks to receive and send money.
This means that we hold funds in bank accounts that are local to that fund's currencies.
Meaning that USD balance is help in a US bank. This is also the reason your USD balance account details will have both local details and SWIFT payment details."
So the answer is that USD amounts are kept in US banks... only necessary to report the Euro side.
Thanks for all the help! Done.
