@mattmcnutt I found this firmware update for my phone which was released on the same date as the update that won't install. This might be the issue :D.
Figured it out. It wasn't the firmware. I had to uninstall Magisk before installing the LineageOS update.
Err... reinstalling Magisk after the update broke it again. Guess I can't use Magisk.
LineageOS with Magisk finally working. I had to flash a different kernel on my phone to get it working with the latest LineageOS.