> I've been advised that there's an income tax exemption for "hobby income" ... up to 10k USD per year

If you're talking about the US, I think you're wrong: you need to declare all your hobby income, and since 2018, you can't deduct hobby expenses.

See e.g. https://www.creditkarma.com/tax/i/hobby-income-taxed https://turbotax.intuit.com/tax-tips/self-employment-taxes/4...

Ah yeah, looks like things might have changed since I got that guidance.

That's annoying. Would be nice if they didn't keep changing things around all the time.