well this just show you dont uses the phone much

cause from data collection
your phone is kept in sleep mode a lot of the times

so by calculation
turning on the stamina mode calculated to be >15 days
if you didnt uses the phone at all

any usage will change to calculation at real time or some time delay

the main culprit for me is the wifi always on
when i turn on the wifi always on, the phone refuses to go into deep sleep

now the best i can do is to turn off wifi when sleep
and the phone can get into deep sleep
thus wakelock lessen

in reality,
rooting of xperia z is not officially supported by Sony

earlier FW have exploit loopholes to enable the rooting process
this exploit was plugged in the 253FW onwards

thats why the rooting process still requires you to use Flashtool to flash old exploitable FW component - root- reflash back new FW component

I use backup like Titanium Backup and previously OBackup

The backup is on the apps you downloaded so when in new FW you can easily reinstall the apps without connecting to the net

backing up and restoring system apps/data is not recommended as the system data might be different between FW

Issue i seen with TiB is Line app
where restoring back the app without even data will FC the app
only after clearing data/cache that the app start back normal