ну вот что на бельгийском сайте написано: https://www.kunstenloket.be/node/6043

You work in a EU country as a self-employed person and come to work in Belgium

Do you live in an EU country but do not hold the nationality of an EU country or of Iceland, Liechtenstein, Norway or Switzerland?

Then you must apply for a professional card.You do not, however, need a professional card if:

as an artist, you provide services for your profession and do not stay in Belgium for more than three months;

or if you are married to an EU citizen.

(There are other exceptions, but they are less relevant.)

You need the following documents to apply for a professional card:

certificate of good standing;

an attestation proving a basic knowledge of business management or a certificate showing that you are exempt from having to provide such evidence.You can request this attestation from an enterprise counter (ondernemingsloket).

a CV; a business plan in which you outline the activity you wish to develop.

Короче походу только через гмбх окучивать можно евространы. Как фрайберуфлер только внутри Германии.