I would suggest upvoting this topic:

https://nintex.uservoice.com/forums/229406-2-nintex-forms-for-sharepoint/suggestions/5208511-document-sets-support-new-edit-document-set

But it looks like you may have already. Unfortunately we can't give you any more information that what you've garnered from that particular post!

Hi Andreas Hack‌. Sorry you have had to wait for this feature. We do see it as the most requested feature on User Voice for Nintex Forms for SharePoint and comes up with my customer conversations.

I am pleased to say that we are currently looking into Document Sets (New/Edit) page. It requires a fair amount of investigation and testing as its not a standard list item form but will keep User Voice updated as we progress.

Cheers,

Euan

It's here

https://community.nintex.com/blogs/NintexProductBlog/2017/11/22/whats-new-document-set-forms-now-available-with-nintex-forms-for-sharepoint