In Nintex for O365 there is an option to use SharePoint Lists in a drop-down or as radio buttons with it being filtered based on an expression (that can reference the value of other Nintex fields in a complex expression or variable using intrinsic functions) with the ability to show unique values only.
The idea is to ensure the implementation of Local Lists (stored in the tenant and does not rely on SharePoint at all) with filtering and unique value presentation capability.
This way it's very fast, doesn't require SharePoint nor SQL, and most importantly, it supports cascading listboxes. For example with 3 listboxes you can select Country --> Province --> City. Selecting the Country will cause the Province listbox to show only provinces from that country. Once the country and province are selected, only cities from there are listed. Changing the province choice will clear the city choice. Changing the country choice will clear both the province and city choices.
Euan Gamble's (former Nintex Forms senior product manager) video on the List Lookup control from October 1, 2020 shows how it works in Nintex for O365. This is a request for feature parity to implement the equivalent in the cloud with no dependency on any external data sources outside of the tenant.
The link here takes you to the exact point in the video that demonstrates in Nintex for O365 for what needs to exist in NWC: https://youtu.be/hEiZREoKVF0?t=357