A well known developer that goes by the name of dmitrygr is currently working on bringing the latest Android OS version to Nexus devices that failed to get the OTA update.
The developer has released a pre-built AOSP Android Marshmallow image for the Nexus 7 (2012) model. The first release was for the Wi-Fi variant of the device codenamed “grouper”, but now it supports also the 3G model that’s codenamed “tilapia”.
According to dmitrygr, the ROM is fully functional but in order to install it on your Nexus 7 (2012) device you will need to wipe out all data, which means that you will also need to create a backup of your system before you go ahead with the installation.
HINT: Keep in mind that flashing the incorrect build might damage your device, so install the custom ROM at your own risk.
Installing Android 6.0 Marshmallow custom ROM By dmitrygr On Nexus 7 (2012)
First of all, you will need to download to your computer the TWRP flashable.zip, Android Marshmallow and either grouper-fastboot-images.zip or tilapia-fastboot-images.zip from dmitrygr’s website. At the same time, you will need to download the Android Marshmallow Gapps.zip file from the internet, but this file is hosted by many other websites.
Now, connect your Nexus 7 (2012) to your computer via the USB cable and copy the zip files to your tablet. Unplug the tablet from your computer and boot it into the bootloader mode. If the tablet is rooted, you will need to use the quick boot app to boot into the bootloader mode. However, if the tablet is not rooted, you will need to switch it off and press and hold the Power+Volume Up buttons to make it load the bootloader mode.
While in recovery, you will need to create a backup of the current tablet setup, which will allow you to restore the tablet anytime you want (in case anything goes wrong during the Android 6.0 Marshmallow installation).
After that, you will need to perform a factory reset by selecting Wipe->Advanced Wipe and select the following: System, Data, Cache and Dalvik Cache. Once you’ve completed the factory reset, go to the main page of the bootloader and select “Install” and look for the grouper-fastboot-images.zip or tilapia-fastboot-images.zip file that you’ve downloaded. Once the installation is complete, you will need to install the Android Marshmallow Gapps.zip and after that, you will have to reboot the Nexus 7 (2012).
Once the Nexus 7 (2012) boots back, you will notice that it runs on Android 6.0 Marshmallow custom ROM by dmitrygr.
Have you tested the custom ROM on your Nexus 7 (2012)?