How can I get root or TWRP without being able to enter any recovery?

I have a phone running Android Marshmallow (6) and apps constantly crash. Most apps I would like to use do not support Android 6. I would like to update to a custom ROM or a newer version. - adb and fastboot work - fastboot mode and recovery mode do not work - the device does not have too How can I proceed? I find it's sort of chicken and egg problem with no recovery and no root. To be clear, I can't even get into stock recovery. The phone is Xperia Z2. Flashtool works but I can't find any newer versions of FTF files.