Old question, but this is my beloved Nexus 7 3g (2012), "tilapia". Please note that WiFi only version is named "grouper"
First step, install TWRP recovery; the official guides forgot to mention that you have to disable the fastboot lock too, command:
fastboot oem unlock (and then select Yes on the confirmation screen, on tablet)
Nexus 2012 TWRP installation details here
Then I have installed on my Nexus the Android AOSP (vanilla 7.1.2. andorid) with tilapia/grouper support released by AndDiSa on XDA forums.
Main thread is here.
Now it works really fast, I can use the web browser, play and see movies with no hiccups.
I quote my "success story" form XDA forum:
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
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, is a Nexus) released
defective OS versions for manufacturer-planned-obsolescence