I'm not sure if this one has been mentioned already, but there should be option to demand fixed tribute. For instance, 10 golds per turn. This could be, for instance, part of peace treaty, ie the losing nation has to pay money to winning nation. Fir instance, Berber Nations lived, towards the end, almost entirely off huge tribute other nations paid to them to be spared from their piracy.

Downside, of course, is that this kind of pirate nation could yank science to be 100% and pay all building keepup costs from tributes, and this isn't correct. Maybe this would only work in Monarchy, Despotism and such primitive goverment types?