Luxembourg
Reported as self-employment income on the steep progressive scale (up to ~42% plus solidarity surtax) with self-employed social contributions ~24-25%. Business income, not capital gains.
Self-employment / commercial-or-independent-profession income, not capital gains.
Progressive PIT ~0-42% plus solidarity surtax (7-9%), so top effective ~45.8%; tax-class dependent.
Self-employed social security ~24-25% (pension 16%, health, dependency, accident) up to a high ceiling (5x minimum wage).
Register as independent; profits taxed on the progressive scale after expenses. No flat small-business income-tax regime.
USDC for services is ordinary professional income at EUR value on receipt; speculative disposal gains within 6 months taxable, long-term private gains generally exempt.