Does anyone credit the Romans for this? The Romans were very much distrustful of democracy preferring a mixed constitution (that put the majority of power among the wealthier classes).

I think you might be mixing up republicanism and democracy. While the Roman State, England, The Dutch Republic and Venice had some institutions that had some forms of democracy, they could hardly be considered Democracies. Often, it would even be hard to call all these countries republics.

England's(United Kingdom) transition from a Monarchy to a Democracy has been a 700+ year process that I would argue is still not completed. In fact in the UK, Anglican bishops and hereditary nobility still have seats in the upper house.

Venice was basically a joint operation of a class of aristocratic trading families that picked and filled government positions.

The Dutch republic was a confusing mess of a government. With semi-hereditery military leaders, unclear government positions and powers. Half the government wanted a full republic and the other half wanted to institute the Princes of Orange as permanent rulers.