Belgium
With another main job, usually ~33% on 'diverse income' (no social contributions). If it becomes the main, regular activity, reclassified as professional income (25–50% + ~20.5% social).
Either 'diverse income' (occasional, ~33%) if you have another main job, or professional self-employed income if regular/organized. Fact-dependent.
Diverse income ~33% + communal surcharges (~35%). Professional income: progressive ~25–50% + surcharge.
Diverse income: none. Professional income: self-employed social contributions ~20.5% of net (reduced if complementary).
If professional, register as indépendant/zelfstandige, join a social-insurance fund, possibly VAT. No fixed flat tax on trading.
USDC valued in EUR at receipt; ~33% if diverse income, or professional income if reclassified.