Freelance that pays €750/mo, best way to legally do this?

Person lives in the Madrid community. Never worked before, it would be the first job from a small company that is located outside the EU. The payments would be paid via Wise or similar platforms already in euros into their ES IBAN bank account. The freelance could go on for just 2-3 months or be something longer like 14 months. No way to know.

What's the easiest and cheapest way to declare that money? Autonomo seems expensive and lots of red tape, is it the only way?