Would you also consider some sort of third way / mixed option, where a pre-order of the paper book also 'throws in' the digital version? And then you embargo as you describe for a while? So that the sequence goes:
- Month -1: Pre-order paper book, get digital version (free, or at cost, or just @ normal price, whatever makes economic sense)
- Month 0: Paper book available for regular purchase, digital version not available
- Month 0 + x: Digital version now available too
I don't pretend to understand the economics of self-publishing, nor is it my business to have any insight into what your margins are on either version... just wondering aloud as a fan.