Have you considered Document Sets where a document set would be used to host the files normally attached to your form?

Can't work out how to make that work.

And now this is separate to attachments in repeating section, however it would be good if I could work out how to copy all attachments from the form item to a document library. 

At any rate that doesn't resolve attachments in repeating section problem.

The idea with document sets is to get rid of the requirement for a repeating section with attachments. The document set holds the metadata and inside you upload the files (you intended to attach in your repeating section). The files can inherit the docset's metadata as well as have additional metadata.

Btw: a document set is a special folder that can hold metadata. The latest version of Nintex Forms also support document sets, so you could give it a richer content/look 'n feel.

Thanks Jean-Pierre Huls. I'll investigate document sets. 

I'd say content types will not help you. with several content types on list/library you can still submit an item of a single type at one moment.

I'd be resistant to building such big form. apart other reason (especially performance), you say you want to run it on mobile.

you say you have some 10 controls per repeater row. you will hardly manage to fit 10 controls (and their labels) on single mobile screen. so assume at least two screens per repeater row. so for 32 repeater rows it sums up for 64 screens. plus some controls for common info, some wasted space..., you are up to 70 screens.

ask your customer whether they are willing to scroll and fill in 70 screens on mobile...

I'd rather approach it with two master and details lists and simple forms for them.

No solution to this one. Attachments just can't be in repeating sections.