I remember just another thing with this devilish thing:
Don't remember if this issue is present with the latest versions of TWRP and CM, but invoking TWRP at boot could brick the phone. Could only reboot into TWRP from CM, perform a backup and restore that instead of a clean install after the next brick. Requiring to go through the D/L mode thing with stock rom, as Fastboot wasn't working.
Use the tool from LG service. For my G6 there appeared a completely rewritten tool one year ago, required for the Oreo update. I think it won't support old models, but it might also work and provide a fix for the Fastboot issue.
Don't remember if this issue is present with the latest versions of TWRP and CM, but invoking TWRP at boot could brick the phone. Could only reboot into TWRP from CM, perform a backup and restore that instead of a clean install after the next brick. Requiring to go through the D/L mode thing with stock rom, as Fastboot wasn't working.
Use the tool from LG service. For my G6 there appeared a completely rewritten tool one year ago, required for the Oreo update. I think it won't support old models, but it might also work and provide a fix for the Fastboot issue.