As to who funded and bears a disproportionate amount of responsibility for the Dutch slave trade I found this a while back when I was doing research on another issue:
'In the early decades of the seventeenth century the Amsterdam stock exchange perfected its regulations, services and products to make it one of the driving forces of the Dutch economy. Each day, from midday until two in the afternoon, 4,000 investors, speculators, merchants and middlemen crowded together in the courtyard of the stock exchange to carry out their business. It is significant that on Saturdays only 2,000 people were at the stock exchange, that being the day the Jews did not work.'
Alberto Guenzi, 2006, 'European Expansion in the Seventeenth Century', p. 73 in Antonio di Vittorio, 2006, 'An Economic History of Europe: From Expansion to Development', 1st Edition, Routledge: New York
Incidentally 'the Portuguese' was a common way of talking about Sephardic jews who had become Marranos (professing Catholicism keeping jewish ways in secret). Albert Hyamson talks about it in his history of the Sephardim in England.