I had never heard of it, but billmonk seems to me more for personal use. MoneyMonk is accounting software targeted to freelancers and people running a company without employees (ZZP is the Dutch abbr), including tax reports etc, creating invoices.
Im freelancer in the NL and i'm using MoneyBird, for dutch / english freelancers. What do you offer more than MoneyBird? Open API || a link with dutch banks?