It comes also with a bugfix (quite mysterious one.. indeed) that in few cases causes the Config obj to be seen as NoneType.
GitHub page here:
https://github.com/ftarlao/check-media-integrity
In order to install a new Android version you have to copy the Android OS image, optional: gapps zip package, optional fix packages and rooting packages in the Nexus SD or
internal storage (I usually use /data/media/0/Download/ folder). When running TWRP, the Nexus (connected with USB) is accessible by PC with the adb command or,hopefully ,like USB storage (or MTP).
In detail, I suggest to install the Android AOSP 7.x.x AndDiSa release (xda thread with instructions):
https://forum.xda-developers.com/nexus-7/development/rom-android-7-aosp-grouper-t3467514
Please note: for the proposed SetupWizardFixCrDroidNexus7.zip fix was mandatory, I think you need to use it too.
Have fun,
Quoting my "success post":
I have successfully installed the aosp_tilapia-7.1.2-ota-eng-20181112.ds.zip on my Asus Nexus 3g 2012 (tilapia), I also installed open_gapps-arm-7.1-pico and Magisk v17.3.
I installed your Android release, by using TWRP-3.2.3-0-tilapia, over the Asus original fully updated Lollipop. In fact the most tricky part was installing TWRP
During installation the initial OS configuration failed, multiple times (I tried with reboots), just during the user account selection. Using the suggested SetupWizardFixCrDroidNexus7.zip fix, fixed the problem, and I was able to complete the device configuration.
Finally:
My Nexus is back to life, is very responsive, browsing and playing movies are very fast...
I have not tried all the functions (not tried camera yet).
With official Lollipop my Nexus was simply unusable, u-n-u-s-a-b-l-e, I used to think that my beloved Nexus was simply too old for the new applications. I think now that Asus(or Google, it is a Nexus device) released defective OS versions for manufacturer-planned-obsolescence