Does the Netherlands have semi-pro leagues too, where there are some pro players and the rest of the team is amateurs?

AFAIK the lowest paid players in the UK make about 20k pounds per year. For many of them, they are basically playing for the love of the game at that point, because they could be making more money doing just about any other job. Sometimes on semi-pro teams, the pro athletes make less than their amateur teammates, because the amateur teammates have high paid office jobs. But, as you said, sometimes those office jobs are a bit suspect.