Because vocoderism seemed to stopped working on his Android M Grouper firmware and the autorotate bug was still an issue I had a look for another Android Marshmallow firmware that worked well on the Nexus 7 (2012). I found a Pure Nexus version for the Nexus 7 that worked quite well.
The Firmware is based on AOSP Android 6.0.1_R13 and is supporting f2fs and comes pre rooted. The autorotate bug seems to be fixed there.
Thread: [UNOFFICIAL][ROM][GROUPER][6.0.1_r10][EXT4/F2FS]★ The Pure Nexus Project ★[28/01/16]
From this point on you proceed at your on risk. Warranty will be void flashing a custom Rom.
Each link leads to the needed file for each step. Download the files prior to step one and store the files on your tablet. This article doesn’t cover the root process. I suggest at least TWRP 220.127.116.11.
- Download the latest image (https://www.androidfilehost.com/?w=files&flid=37911)
- Also download the Gapps (http://opengapps.org/ ARM 6.0 Nano)
- Move files to the Nexus using a USB cable.
- Reboot into the new recovery
- Perform a factory reset ( May not me necessary if coming from another Android M )
- Flash Pure Nexus image
- Flash Gapps
- Reboot and setup your device.
The first couple of minutes the tablet may feel a bit slow. But after all background processes finished the gain in performance is quite noticable. Also the battery drain is reduced. As mentioned above the autorotate issue is gone as well.