guess i was a bit paranoid
thx for your elaborate answer!
It appears I have this bootloader:
LA3.0_L_15.4
Also, my device seems to not have received Marshmallow, so it must have been branded before I bought it.
I followed the bootpain guide, but still 23.1.A.1.28 is the newest I can select.
I'm flashed OmniRom 8.0 following your instructions.
first boot.img, then twrp
(btw, the md5sum is 3df6806c9fcee4b356d7def1b601930d for omni-8.0.0-20171101-z3-nailyk.zip, I hope - it's missing in your guide

)
My notification light is unfortunately broken by a Media Markt "repair" guy, so i'm not sure I press vol- at the right time...
Now it only boots until the sony logo shows up and that's it.
It's not surprising that my rom doesn't boot, but recovery would be nice... did I miss something?
can I just flash omnirom with fastboot as well like
?