For those with the boot up issues, did you create a bootable flash drive per this link: Créer une clé USB bootable

and THEN follow the install procedure for High Sierra at this link: How to Install macOS High Sierra

I didn’t replace my hard drive, but those seem to be crucial steps prior to replacing it.

This document is also referenced in a comment in one of the earliest steps of this guide. https://s3.amazonaws.com/ifixit-assets/M...

Absolutely great guide for logic board replacement. iFixIt.com was also great in overnighting a replacement to the first logic board they sent me because it was missing the fan connector. Love the warranty on their boards. Worth the extra money vs buying on eBay, etc.