Much thanks to steadfasterX for the files and helping me to unbrick my device this way.
This guide is for devices in Qualcomm 9008 mode (hard brick)
1. Download this archive.
2. Install every driver/runtime and QPST from the "software_drivers" folder (do not install virtualhere_client files, those are not needed for unbricking)
3. Extract "flashing" folder.
3. If your device is not in 9008 mode already, force boot to it (tutorial in "stuff" folder in archive, or look up LG G4 EMMC shorting)
It should show up something like this when properly in 9008 mode (QHUSB_BULK in device manager means drivers are not properly installed, try again)
4. Once your device is in 9008 mode, Open QFIL, disconnect it from the computer, remove the battery and connect it back to the computer with battery removed.
5. Proceed to select the files from the "flashing" folder you extracted in QFIL, then press the "Download" button.
If all goes right, it should flash properly and your device should be able to enter download mode, then flash stock .kdz with LGUP or any other software like usual.
According to steadfasterX, this could work on other H815 variants, but DO NOT proceed if you are afraid of bricking the device completely.
For further support join #Carbon-user IRC channel on freenode (direct link)