[G Pad 8.3 Q&A, Help & Troubleshooting] rizzin:
I'd like to write about my experience too as it took me some time to finally get the device up and running.

I will describe what I wanted to do, what mistakes I've made and what should've been the correct way.

The device itself is LG G Pad 8.3, Verizon LTE variant aka vk810

Initally I received it in a bricked state, on launching it showed the "Secure booting error" message and that's it.

1) Unbricking

The goal was to install an official LG system image and restore the tablet to a working state.

Basically, I did everything according to the steps outlined here: https://xdaforums.com/lg-g-pad-83/general/vk810-4g-reliable-to-root-install-t3283027

Installed LG drivers

Downloaded the LG Flash Tool 2104

Downloaded a KDZ image

Plugged the device in the download mode into the PC and installed the KDZ using the flash tool

At that point I got a fancy idea to install a non-stock Android on this tablet.

2) Lineage OS

I chose this OS for my tablet, because it seems to be the current standard for non-stock Android installations.

I followed these installation instructions: https://wiki.lineageos.org/devices/v500/install

BEWARE: that page is for v500, so it won't work for vk810.

Lineage doesn't support vk810 officially. A mistake I've made was assuming that I can just roll along with a v500 installation. Actually, you kinda can, but for me it resulted in Wi-Fi not working and my device's MAC address being 02:00:00:00:00:00.

3) Rooting

I was quite confused here too: there are different rooting methods, and some of them can be applied in different ways.

Ultimately I found Stump to be the most comfortable for me: https://xdaforums.com/lg-g3/orig-development/root-stump-root-lg-g3-sprint-verizon-t2850906

It works fine on this device and uses a very straightforward process: download APK to your PC, connect the tablet to the PC, "adb install <filename>.apk", then launch the app and that's it. It had to go through the bruteforce process to root, which took several minutes.

4) TWRP

I don't remember what exact TWRP I installed initially, but it was a 3.x version, and I encountered an issue where it couldn't mount any of the tablet's partitions. That was quite a pain in the ass.

For a while I resorted to using an older version, 2.7.x, and it worked fine.

Ultimately what I'm the most happy with is this: https://xdaforums.com/lg-g-pad-83/general/vk810-twrp-3-1-1-0-unofficial-t3691551

The lesson learned here is that, again, you need to make sure stuff you install is compatible with your device.

Ultimately, after making mistakes at each step of the process, I figured out everything, and just followed the installation instructions on the Lineage page I linked earlier.

* Flash stock rom

* Root using Stump

* Enable USB debugging on the tablet, install Stump via adb and root the device

* Install vk810 variant of TWRP 3.2.1-0 via adb

* Reboot to recovery (turn off the tablet -> hold Volume Down + Power -> you should get a factory reset menu after the LG logo -> select factory reset, then confirm it and you'll be booted into TWRP. That factory reset is fake after you have TWRP installed)

* Wipe everything

* adb push the lineage / lineage's su / opengapps onto the device

* Install lineage, then su and opengapps and you're good to go

Main lesson learned: it is CRUCIAL to make sure stuff you install is compatible with your particular device. Do not assume that you can use another version, even if it is made for a device of the same family.