[LG G5 ROMs, Kernels, Recoveries, & Other Developme] shadow52:
I think all my issues are resolved now. Clean flashing seems to have sorted out the rapid battery drain. I have had one boot where the phone became unresponsive - there was a notification that the SD card content was being reviewed and the launcher didn't appear. It was replaced with an animated progress bar "Phone is Starting...". The only accessible actions were to lower the notification shade and the power menu. I'm not sure if the SD card is the issue, but I think that's probably a red herring - I suspect it's more likely the intermittent issue with the first fingerprint unlock that others have described. The issue went away after a reboot (the SD card notification always appears and the card, exfat portable storage, isn't available for several minutes until it completes). UPDATE: I'm now pretty sure it was the SD card, maybe now I've learnt not to jump to conclusions before testing. See my next post for update.

Many thanks again to [MENTION=2881807]x86cpu[/MENTION], both for a fantastic ROM and the help getting the issues sorted. With a clean flash everything's just perfect. I had been thinking the only way to get the latest security updates and fully working features was to buy a new phone that I don't really want (no removable battery) and can't really afford right now anyway. Thanks also to everyone who posted suggestions.

My updated steps and possible pitfalls, moved here from an earlier post:

I should probably post things that I believe can cause problems and answer my own earlier unanswered questions, both for myself to refer to later and anyone else who's struggling. There's a lot of assumed knowledge about flashing on the forums and I think that can be an issue. For myself, although I've been visiting XDA devs since the windows mobile days, I've only learnt what I needed to learn to get a ROM running. I only pick up new knowledge when I encounter a problem and I've probably forgotten a lot of things.

Anyone who spots any definite mistakes, please let me know and I will update.

Notes on Possible issues:

If you experience any issues on first boot, I recommend removing the SIM and SD card. For my device, the SIM seems to prevent the initial setup wizard from running.

Two issues that may cause a TWRP bootloop after flashing:

Installing the TWRP app as a system app.

A buggy version of OpenGApps.

Notes on Baseband, Boot and Bootloader:

These essentially contain the software to run the mobile radio devices (modem or baseband) and start the phone before loading the ROM (boot/bootloader). Normally, these need to match the major version of android/lineage you are installing. For x86cpu's ROMs here (15.1 and 16.0) it is a special case and Nougat versions should be used. UDPATE: I'm currently using the latest baseband and bootloader from stock Oreo (H85030f) without issue.

Pre-requisite going from Stock, clean flash (factory reset apps and data, remove installed ROM):

Go to wipe. "Format Data" (not wipe).

Reboot to recovery

Repeat the above steps if you see any errors/warnings (highlighted in red).

Then select advanced wipe and choose system and cache. (Previously I wiped everything except external sd and usb-otg, but now believe the steps here are correct.)

Proceed to "Pre-requisites".

Pre-requisite going from Lineage 14.1 (dirty flashing without removing apps and data):

Flash to-x86cpu.zip.

Proceed to "Pre-requisites".

Pre-requisites:

Always flash 15.1 first (I think others may disagree, but my own experience and other forum posts lead me to believe this is much more likely to succeed than a clean flash). UDPATE: I am running a clean flash of 16.0.

Flash 15.1

Wipe cache/dalvik

Flash GApps if desired

Wipe cache/dalvik

Flash Magisk if desired

Wipe cache/dalvik

Reboot to system, configure.

Reboot to recovery

Proceed to "Install".

Install:

Flash 16.0

Wipe cache/dalvik

Flash GApps if desired

Wipe cache/dalvik

Flash Magisk if desired

Wipe cache/dalvik

Reboot to system