I’m guessing that since my phone had Nougat installed originally (7.1…) and the bullhead images are Orea based (going by the naming convention of the image filename prefix) that I need to figure out how or where on my 5X that information is kept and to somehow overwrite it?
Not knowing a whole lot about this stuff it seems like there is a location on the device for storing the last known factory image (“vendor.img”) and that if it doesn’t align with the image used to build the boot image then the system will fail to boot. I’m sure there are many gaps or problems with the above statement.
At any rate the gist above covers most everything. The high level is that one will need to download the very last factory image from Google (Dec 2018 for Oreo) and unzip the image to retrieve and copy to the device (/sdcard) the “vendor.img” file. Once that is complete you then need to use TWRP to copy (or rather “install”) it from /sdcard (I’m assuming that is an internal sd card since there is no removable sd card on 5X). This is done in TWRP by Install -> Internal storage -> select vendor.img -> swipe to install.