Successfully installed Mojave using a created USB stick with this guide.
Sadly, I can't get my graphics (GTX 970) to work. System reverted back to the default MacOS drivers after the update, after which I had to use Nvidia-update.sh (with the -f flag) to reinstall the web drivers.
System now outputs 4K correctly, but with many glitches and lag. Dock and menu bar are opaque rather than transparent. I tried Lilu + NvidiaGraphicsFixup, to no avail.
On a side note: I can't seem to find the High Sierra Installer in the App Store, so I can't downgrade quite yet.
EDIT: Found I could restore to 10.3.5 from my Time Machine backup, using the restore partition. Trying that now, will report back.
EDIT 2: Backup from Time Machine restored just fine, even on hackintosh. Just had to enter icloud password again and was all set.