I have both Nebo & Goodnotes... they have different usage.

The feature you are talking about is indeed very much useful BUT the UX requires A LOT of work - Nebo's UX is not great & you need to get used to it.

So I am not sure it would be worth the effort... or it would bring much frustration to the usage of Goodnotes.

Why don't you use Nebo to convert & then copy / paste into Goodnotes?
Having a super app which does everything is a pleasant thoughts but not necessarily THE solution.

You could request Nebo to copy so much of Goodnotes features in this case... or maybe have you? :)

Actually the keeping of Outline shouldn't depend on Flattened or Editable pdf... it should be there, period !

Keeping the Outlines on Editable docs doesn't make real sense !

We export flattened pdfs so we can consult our docs for later reading and copy / paste from any other computer / device. Outlines make sense in THIS CASE.

When you export editable pdfs, you can't select text to copy / paste and the doc is way heavier - this is true for BOTH handwritten text & text boxes !

The files are, in this case, useless for consult. So why would I need outlines in this situation?

This choice just doesn't make ANY sense !
Maybe it was easier, technically speaking, but if not I see no point in this choice !