Spain
Taxed as self-employed business income on the IRPF progressive scale plus mandatory autónomo contributions. No low flat regime — effective burden is meaningful.
Self-employment / business income ('rendimientos de actividades económicas') in the IRPF general base — not savings-base capital gains.
Progressive ~19% up to €12,450, ~45% over €60,000, 47% over €300,000 (state + region; varies by region).
Register as autónomo → monthly RETA contributions on an income-based scale (~31.4% of a tiered base).
Alta in Hacienda (modelo 036/037) + RETA. Quarterly IRPF prepayments (modelo 130) + annual Renta. No low flat-rate regime for this.
USDC is income at EUR value on receipt; that value is the basis, so a later conversion can trigger a savings-base capital gain/loss.