(1) the credentialism of the Spanish job market has gotten there too, and even though I know enough about painting, cleaning, gardening and small repairs to work in that area, an FP (for everyone outside Spain: the equivalent of a trade school degree) is needed to secure a position,
Maybe I wasn't clear enough. The first years my argentinian friend was working as freelance for people with low income, doing petty works (painting, fixing stuff, sticking advertisements), no invoices, no credentials, no taxes, in the informal market. That way he made some money, some clients, and when he learned the job enough, he got a plumber certificate (you may get one by passing an exam, which he studied in his free time) and in time he paid the freelance fee and now he can sign his own certificates and give invoices as required.
I know there are free exams to some FP (professional formation) modules that don't require you to attend to classes, just passing the exams, which you can prepare with some books. It helps if you already know the trade.
I don't say that you have to follow his steps, maybe the market has changed in the meantime. I'm just saying that you can look for what the people is actually demanding and try to find something you think you can make some money. Preferably something suitable to your skills. I know technicians are in high demand in my city.
However.
I have another friend who is tutoring bachelors, (profesor particular), he is still unable to pay the freelance fee. Might be earning 700 euros per month. That's not much, but then he is living in a very cheap and small city, and his wife has a much better paid job. He is not a good example, but he is content because he can schedule the classes and care for their children. Actually he's taking the role of househusband (half journey job, caring for the house and the children). My point here is that teaching is not very profitable at this level.
Subnote: You need one and a half salary to pay for a home. If you only have one job, you may just be able to afford renting a room.
Subnote: Madrid has better paid jobs, but also more expensive lifestyle. Yes, there are more opportunities, but unless you plan on climbing in a big company, where you get the really well paid jobs, or become really successful with your own company, probably you will make a better life in a middle-sized city, like Sevilla or Valencia, where they are still open to foreigners, especially if they are honest workers.
Final note. If you follow the crowd, expect to be underpaid and abused. The same goes for the would be farmers around this forum. Trying to be a regular (small) farmer is a recipe for poverty.