How I Stopped Hating My Xperia Z3C

This is mostly a documentation thing. My Xperia Z3C decided to stop functioning like a nice smartphone after the Marshmallow update Sony released for it. My battery went from a nice day and a half or so with light usage to less than 20 hours (and just today it couldn't even pull off 12 hours) on standby. Unacceptable. So, off to XDA I went.

First I followed this guide which is long, but thorough. Unfortunately something seemed to have gone wrong as, by the end of it, I had lost my root access (which I want so that I can use some of the better battery/optimization apps out there). I'm going to write down what I'm doing till something works. Hopefully.

Flashtool

So I already got Flashtool installed. I got it to download the global version of 23.0.A.2.93. Flashed it on the phone.

Rooting

Enable Developer mode (Settings > About > Tap 7x on Build Number) and then enable: USB Debugging and Mock locations.

I know I had ADB installed and Windows installed some drivers when I plugged it in automatically. I unzipped this rooting tool and conncted my phone to the PC, enabled Airplane mode, then ran the install.bat. The phone does junk and eventually reboots. You might have to do it twice apparently, but it'll work.

Root successful.

Backup DRM Keys/TA Partition

Steps 22 - 26 tell me about this. I already did it so I'm assuming I'm good for now.

The Upgrade

I found a supposedly flashable, pre-rooted Marshmallow (23.5.A.0.570), so I moved that zip into the / of the internal storage.

Now after getting the Z3C version of Dual Recovery I do a quick edit to something. I must run adb shell, su, and finally mount -o remount,rw /system before I run the install.bat (choose option 1).

The phone will reboot and boot into the recovery thing. I did a Full System Wipe, then install the zip. I did another wipe after completion, then powered it off. I turned the phone back on after removing the USB cable.

I now have a working Marshmallow with root access.

Post-install

I installed 3 apps recommended by Reddit to detect battery eaters. I'll continue updating this post as I learn/do more stuff.