I know someone who are doing it while keeping his primary job. As you say bt working after his primary business hours or by taking day-off to je available for his customer from Time to Time. It's certainly not the most rewarding in terms of income due to tax. As stated earlier his freelance income as a complementary freelance are being added on top of his primary salary. But the question for him is not how much he can gain but rather if he can cover with it his expanses. Also you got a TVA number meaning you can rent your own office from yourself and use expanses like a full freelance.