You can progress to merchant shipping:

With your GZV Certificate of Competency, you can take an additional training course at FIRDA in Urk.

Training institute Firda in Urk offers students with a GZV certificate of knowledge and competency the opportunity to obtain a qualification as mate all ships or Maritime Officer Small Ships Nautical within one year (and at half the tuition fee), including a 90-day internship.
The internship is not 360 days because, if you hold a sailing commercial qualification, you only need to demonstrate three months of sea time for your second qualification.