I do understand that it makes sense for the document to pop back into central view when dragging to the side, but when handwriting at the document’s edge, this becomes an annoyance.

How about a new action/feature where the documents still pop back into central view under normal circumstances, but when the user really drags the document far off to the side, the document stays there at the side of the screen. Now the user can handwrite comfortably, with his hand resting on the black part (background) of the screen, and the document on the other side of the screen. And when the user wants it back into the default ‘pop to center view’, just slide/flick the document back towards the center and it’ll be back to the default behavior.

Without this feature, handwriting on a document’s edge will result in a person’s hands needing to be held off the side of the device’s screen, which is not good. With the ability to keep the document on a particular side of the screen, the user’s hand can rest on the other side of the screen for more comfortable handwriting.

In the first photo, is the current state of GoodNotes where the hand goes off the device surface when writing at the document's edge. The second photo shows the ideal GoodNotes where a document can be set into a particular side (not split view) so the handwriting hand can rest on the tablet screen.