Failed Android 7.1.2 install on LG V500 - stuck on boot screen
Solved it by downloading LG Mobile Support Tool, switching the tableto into download mode (Power Off, then press both Up and Down vlume until it shows download mode), then connecting the USB and after the LG Mobile Support tool recognizes the tablet, go to Options -> Upgrade Recovery.
The tool will download and flash the latest stock image. 
After that I got stuck on boot loop and performed factory reset:


Press and hold Power Button + Volume Down
When the LG logo shows up, release the power button only
Now press and hold the Power and Volume Up button while keeping your hold on the Volume Down button
When the factory reset screen appears, release your hold on all buttons
Follow On Screen Indications


Now I get a fully stock tablet that I can root, install custom recovery etc